m24 hald6891 06 se ch24 - the eye · nose. sometimes a valve “tick, tick, tick” noise is heard...

# 101145 C t PH OH CHET A H ld P N 437 K/PMS DESIGN SERVICES OF

After studying Chapter 24, the reader will be able to:

1. Prepare for the Engine Repair (A1) ASE certification test

content area “B” (Cylinder Head and Valve Train Diagnosis

and Repair).

Aerated (p. 472)

Asymmetrical (p. 448)

Bucket (p. 452)

Cam Chucking (p. 446)

Cam Follower (p. 452)

Cam-in-Block (p. 438)

Camshaft Bearings (p. 438)

Camshaft Duration (p. 453)

Camshaft Overlap (p. 454)

Composite Camshaft (p. 441)

Contour (p. 438)

Exhaust Valve Cam Phaser (EVCP) (p. 463)

Finger Follower (p. 452)

Flat-Link Type (p. 444)

Freewheeling (p. 445)

Hydraulic Lash Adjusters (HLA) (p. 452)

Hydraulic Valve Lifter (p. 467)

Intake Centerline (p. 459)

Intake Lobe Centerline Method (p. 458)

Lift (p. 447)

Lifter Preload (p. 469)

Lobe Centers (p. 454)

Lobe Displacement Angle (LDA) (p. 454)

Lobe Separation (p. 454)

Lobe Spread (p. 454)

Morse Type (p. 444)

Oil Control Valve (OCV) (p. 461)

Overhead Camshaft (OHC) (p. 438)

Overhead Valve (OHV) (p. 439)

Pump-Up (p. 469)

Ramp (p. 467)

Seat Duration (p. 458)

Silent Chain Type (p. 444)

Solid Valve Lifter (p. 467)

Symmetrical (p. 448)

Thrust Plate (p. 446)

Total Indicator Runout (TIR) (p. 456)

Valve Float (p. 469)

Valve Lash (p. 467)

Variable Valve Timing (VVT) (p. 462)

Variable Valve Timing and Lift Electronic

Control (VTEC) (p. 466)

Zinc Dithiophosphate (ZDP) (p. 456)

CHAPTER 24

CAMSHAFTS AND

VALVE TRAINS

CAMSHAFTS AND

VALVE TRAINS

2. Describe how the camshaft and valve train function.

3. Discuss valve train noise and its causes.

4. Explain how to degree a camshaft.

5. Explain how a hydraulic lifter works.

OBJECTIVES

KEY TERMS

M24_HALD6891_06_SE_CH24.QXD 6/11/08 11:08 AM Page 437

438 CHAPTER 24


CAMSHAFT PURPOSE

AND FUNCTION

The camshaft is driven by timing gears, chains, or belts locatedat the front of the engine. The gear or sprocket on thecamshaft has twice as many teeth, or notches, as the one onthe crankshaft. This results in two crankshaft turns for eachturn of the camshaft. The camshaft turns at one-half thecrankshaft speed in all four-stroke-cycle engines.

The camshaft’s major function is to operate the valvetrain. Cam shape or contour is the major factor in determin-ing the operating characteristics of the engine. The lobes onthe camshaft open the valves against the pressure of the valvesprings. The camshaft lobe changes rotary motion (camshaft)to linear motion (valves).

Cam lobe shape has more control over engine perfor-mance characteristics than any other single engine part. Enginesidentical in every way except cam lobe shape may have com-pletely different operating characteristics and performance.Two cam shapes for a small-block Chevrolet V-8 are shown inFigure 24-1.

The camshaft may also operate the following:

■ Mechanical fuel pump■ Oil pump■ Distributor

See Figure 24-2.

CAMSHAFT LOCATION

Pushrod engines have the cam located in the block. SeeFigure 24-3.

This design is called the cam-in-block design. The cam-shaft is supported in the block by camshaft bearings and

FIGURE 24-1 Shape of two small-block Chevrolet V-8 cam lobes.A standard cam is on the left and a high-performance cam is on the right.

DISTRIBUTORSHAFT

OIL PUMP

FIGURE 24-2 The camshaft often is used to drive the oil pump anddistributor, if equipped.

FIGURE 24-3 The camshaft rides on bearings inside the engineblock above the crankshaft on a typical cam-in-block engine.

driven by the crankshaft with a gear or sprocket and chaindrive. Overhead camshafts are either belt or chain driven fromthe crankshaft and are located in the cylinder head(s). Thisarrangement is called overhead camshaft (OHC) design.

CAMSHAFT PROBLEM

DIAGNOSIS

A camshaft with a partially worn lobe is often difficult to diag-nose. Sometimes a valve “tick, tick, tick” noise is heard if the

M24_HALD6891_06_SE_CH24.QXD 6/10/08 12:04 PM Page 438

Camshafts and Valve Trains 439


cam lobe is worn. The ticking noise can be intermittent,which makes it harder to determine the cause. If the enginehas an overhead camshaft (OHC), it is usually relatively easyto remove the cam cover and make a visual inspection of allcam lobes and the rest of the valve train. See Figure 24-4.

In an overhead valve (OHV) engine, the camshaft is inthe block, where easy visual inspection is not possible. SeeFigure 24-5.

CAMSHAFT REMOVAL

If the engine is of an overhead valve design, the camshaft isusually located in the block above the crankshaft. The timingchain and gears (if the vehicle is so equipped) should be

removed after the timing chain (gear) cover is removed.Loosen the rocker arms (or rocker arm shaft) and remove thepushrods.

NOTE: Be sure to keep the pushrods and rocker arms together if they are

to be reused.

Remove or lift up the lifters before carefully removing thecamshaft. See the Tech Tip, “The Tube Trick.”

CAMSHAFT DESIGN

The camshaft is a one-piece casting with lobes, bearing jour-nals, drive flanges, and accessory gear blanks. The accessorydrive gear is finished with a gear cutter. The lobes and journals

To quickly and easily test whether the camshaft is okay,observe if the pushrods are rotating when the engine isrunning. This test will work on any overhead valvepushrod engine that uses flat-bottom lifters. Due to theslight angle on the cam lobe and lifter offset, the lifter(and pushrod) should rotate whenever the engine isrunning. To check, simply remove the rocker arm coverand observe the pushrods when the engine is running.If one or more pushrods are not rotating, this camshaftand/or the lifter for that particular valve is worn andneeds to be replaced.

TechTipTHE ROTATING

PUSHROD TEST

FIGURE 24-4 The camshaft on an overhead camshaft design enginecan be easily inspected for wear or damage by removing the valve cover.

(a)

(b)

FIGURE 24-5 (a) Here is what can happen if a roller lifter breaksloose from its retainer. The customer complained of “a little noise from theengine.” (b) All engines equipped with roller lifters have some type of retainerfor keeping the lifters from rotating.


440 CHAPTER 24


Valve lifters are often difficult to remove because the endsof the lifters become mushroomed (enlarged) where theyhave contacted the camshaft. Varnish buildup can alsoprevent the lifters from being removed. Try this method:

Step 1 Raise the lifters upward as far away fromthe camshaft as possible.

Step 2 Slide in a thin plastic or cardboard tubewith slots in place of the camshaft.See Figure 24-6.

Step 3 Push the lifters downward into the tube.Use a long magnet to retrieve the liftersfrom the end of the tube.

This trick will work on almost every engine that hasthe camshaft in the block. If the tube is made from plas-tic, it has to be thin plastic to allow it to flex slightly. Thelength of the lifters is greater than the diameter of thecam bearings. Therefore, the lifter has to be pusheddownward into the tube slightly to allow the lifter roomto fall over into the tube.

TechTipTHE TUBE TRICK

FIGURE 24-6 Instead of prying old lifters up and out of the engineblock, use a plastic (or cardboard) tube in place of the camshaft and push thelifters down.Then use a magnet to pull the old lifters out of the tube.

are ground to the proper shape. The remaining portion of thecamshaft surface is not machined. See Figure 24-7.

On pushrod engines, camshaft bearing journals must belarger than the cam lobe so that the camshaft can be installedin the engine through the cam bearings. Some overhead camengines have bearing caps on the cam bearings. These camscan have large cam lobes with small bearing journals. Cam

A technician working in a new-vehicle dealership dis-covered a noisy (defective) valve lifter on a Chevroletsmall-block V-8. Another technician questioned howlong it would take to replace the lifter and was told,“Less than an hour”! (The factory flat-rate was muchlonger than one hour.) Ten minutes later the repair tech-nician handed the questioning technician a hot lifterthat had been removed from the engine. The lifter wasremoved by the following steps:

1. The valve cover was removed.2. The rocker arm and pushrod for the affected

valve were removed.3. The distributor was removed.4. A strong magnet was fed through the distributor

opening into the valley area of the engine. (If thevalve lifter is not mushroomed or does not havevarnish deposits, the defective lifter can be liftedup and out of the engine; remember, the techni-cian was working on a new vehicle.)

5. A replacement lifter was attached to the mag-net and fed down the distributor hole and overthe lifter bore.

6. The pushrod was used to help guide the lifterinto the lifter bore.

After the lifter preload was adjusted and the valvecover was replaced, the vehicle was returned to the customer in less than one hour.

TechTipHOT LIFTER

IN 10 MINUTES?

bearings on some engines are progressively smaller from thefront journal to the rear. Other engines use the same size ofcamshaft bearing on all the journals.

Most older automotive camshafts were used with flat orconvex-faced lifters and made from hardened alloy cast iron.The cast iron resists wear and provides the requiredstrength. The very hardness of the camshaft causes it to besusceptible to chipping as the result of edge loading or care-less handling.

Cast-iron camshafts have about the same hardness through-out. If reground, they should be recoated with a phosphatecoating.

Steel camshafts are usually SAE 4160 or 4180 steel andare usually induction hardened. Induction hardening involvesheating the camshaft to cherry red in an electric field (heatingoccurs by electrical induction). The heated camshaft is then

M24_HALD6891_06_SE_CH24.QXD 6/10/08 12:04 PM 40



dropped into oil. The rapid cooling hardens the surface.Camshafts can also be hardened by using the following:

■ Liquid nitriding. Hardens to 0.001 to 0.0015 inch ofthickness

■ Gas nitriding. Hardens to 0.004 to 0.006 inch of thickness

Typical camshaft hardness should be 42 to 60 on theRockwell “c” scale. If this outer hardness wears off, the lobesof the camshaft are easily worn until they are almost com-pletely rounded, as shown in Figures 24-8 and 24-9.

REAR BEARING

LOBE OR NOSE

DRIVE GEAR FOR DISTRIBUTOR (OIL PUMP)

OIL HOLES

OIL HOLES

CAMS

FRONT BEARING

GEAR FIT

KEYWAY

ECCENTRIC (FOR FUEL PUMP IF USED)

OIL GROOVE

TOPPEDHOLE

DIAMETER

BASE CIRCLE

LIFT

LEADING FLANK (OPENING ACCELERATION)

BASE � LIFT TIMING POINT

HEEL OR FOOT

TRAILING FLANK (CLOSING ACCELERATION)

FIGURE 24-7 Cam and camshaft terms (nomenclature).

FIGURE 24-8 Worn camshaft with two lobes worn to the point ofbeing almost round.

WELDED UP LOBE

FIGURE 24-9 Worn camshaft that has been restored by welding thelobes and regrinding the original contour.

NOTE: Rockwell is a type of hardness test, and the c represents the scale

used.The higher the number, the harder the surface.The abbreviation Rc60,

therefore, indicates Rockwell hardness of 60 as measured on the “c” scale.

COMPOSITE CAMSHAFTS

A composite camshaft uses a lightweight tubular shaft withhardened steel lobes press-fitted over the shaft. See Figure 24-10.


442 CHAPTER 24


The actual production of these camshafts involves placingthe lobes over the tube shaft in the correct position. A steel ballis then drawn through the hollow steel tube, expanding thetube and securely locking the cam lobes in position.

CAMSHAFT LUBRICATION

Some engines transfer lubrication oil from the main oil galleryto the crankshaft around the camshaft journal or around theoutside of the camshaft bearing. Cam bearing clearance iscritical in these engines. If the clearance is too great, oil willleak out and the crankshaft bearings will not get enough oil.Other engines use drilled holes in the camshaft bearing jour-nals to meter lubricating oil to the overhead rocker arm.Oil goes to the rocker arm each time the holes line up betweenthe bearing oil gallery passage and the outlet passage tothe rocker arm. Camshaft oil metering holes are shown inFigures 24-11 and 24-12.

FUEL-PUMP ECCENTRICS

An eccentric cam lobe for the mechanical fuel pump is oftencast as part of the camshaft. A mechanical engine-drivenfuel pump is used on older engines equipped with a carbu-retor. The fuel pump is operated by this eccentric with along pump arm or pushrod. Some engines use a steel cuptype of eccentric that is bolted to the front of the cam drivegear. This allows a damaged fuel-pump eccentric to be re-placed without replacing an entire camshaft. Typical fuel-pump eccentrics are identified on a number of camshaftspictured in Figure 24-13.

CAMSHAFT DRIVES

The camshaft is driven by the crankshaft through gears, sprock-ets and chains, or sprockets and timing belts. Timing chains

HOLLOW STEEL SHAFT

CAM LOBES

FIGURE 24-10 A composite camshaft is lighter in weight than aconventional camshaft made from cast iron.

FIGURE 24-11 Hole through a camshaft bearing journal. The holemeters oil to a rocker shaft when it lines up with oil passages in the cam bearings.

FIGURE 24-12 Damaged camshaft bearing support.This overheadcamshaft engine overheated because of an electric cooling fan circuit failure.The cylinder head warped upward in the center, causing a binding of thecamshaft in the bearing.

are not as wide as timing belts, so engines with timing chainscan be shorter. Timing chains often have tensioners (dampers)pressing on the unloaded side of the chain. The tensioner pad is a Nylatron™ molding that is filled with molybdenumdisulfide to give it low friction. The tensioner is held againstthe chain by either a spring or hydraulic oil pressure as shownin Figure 24-14.




TENSIONER HELDWITH A SPRING

(a)

FIGURE 24-14 (a) A spring-loaded timing chain tensioner (also called a damper). (b) Most overhead camshaft engines use a hydraulic tensioner. (c) Hy-draulic tensioners use engine oil pressure to keep tension on the chain. A ratchet mechanism in the tensioner maintains some tension on the chain when the engine isshut off and oil pressure is zero.This design helps reduce noise when the engine starts and before oil pressure is again applied to the tensioner.

DIAPHRAGMSPRING

PUMPBODY

INLETCHECKVALVE

INLETFITTING

FUELCHAMBER PULSATOR DIAPHRAGM

OUTLETCHECKVALVE

PUMPDIAPHRAGM

CAMSHAFTECCENTRIC

ROCKERARM

FIGURE 24-13 One part of the camshaft used on older model vehicles includes an eccentric that is used to operate the mechanical fuel pump.

TENSIONER HELD WITHOIL PRESSURE

(b)

PLASTICCHAIN GUIDES

HYDRAULICCHAIN DAMPER

(c)

The gears or sprockets are keyed to their shafts so thatthey can be installed in only one position. The gears andsprockets are then indexed together by marks on the gearteeth or chain links. When the crankshaft and camshaft timing marks are properly lined up, the cam lobes are in-dexed to the crankshaft throws of each cylinder so that thevalves will open and close correctly in relation to the pistonposition.


444 CHAPTER 24


FIGURE 24-15 Two types of sprockets that can be used on the sameengine.A cast-iron sprocket is on the left, and an aluminum nylon sprocket is onthe right.

CAMSHAFT CHAIN DRIVES

The crankshaft gear or sprocket that drives the camshaft isusually made of sintered iron. When gears are used on thecamshaft, the teeth must be made from a soft material to re-duce noise. Usually, the whole gear is made of aluminum orfiber. When a chain and sprocket are used, the camshaftsprocket may be made of iron or it may have an aluminum hubwith nylon teeth for noise reduction. Two types of timingchains are used.

1. Silent chain type (also known as a flat-link type, orMorse type for its original manufacturer). This type operates quietly but tends to stretch with use. SeeFigures 24-15 through 24-17.

NOTE: When the timing chain stretches, the valve tim-ing will be retarded and the engine will lack low-speedpower. In some instances, the chain can wear throughthe timing chain cover and create an oil leak.

2. Roller chain type. This type is noisier but operates withless friction and stretches less than the silent type ofchain. FIGURE 24-17 Excessively worn timing gear and chain.

Many mechanical fuel pumps operate off of a separatelobe on the camshaft. If this fuel-pump lobe becomesworn, the stroke of the fuel pump is reduced and the amount of fuel being supplied to the engine is re-duced. The engine may experience a lack of power orcut out and miss under load. The problem can also beintermittent, depending on other factors. A worn fuel-pump cam lobe is often found on Ford 240- and 300-cubic-inch inline 6-cylinder engines. Some Ford Escortengines experience a worn fuel-pump pushrod and behave similarly to an engine with a worn fuel-pumpcam lobe.

If a worn fuel-pump cam lobe is suspected, per-form a fuel-pump capacity (volume) test. If the pumpdoes not pump at least 1/2 pint in 15 seconds (1 pintin 30 seconds), then remove the pump and inspect forexcessive cam lobe wear or fuel-pump pushrod wearbefore replacing the fuel pump.

TechTipCHECK THE

CAMSHAFT AND

THEN THE FUEL

PUMP

FIGURE 24-16 Close-up view of two types of timing chains. A silentchain is on the left, and a roller chain is on the right.




Some four-cam engines use a two-stage camshaft drivesystem:

■ Primary: From crankshaft to camshaft■ Secondary: From one camshaft to another

See Figure 24-18.

CAMSHAFT BELT DRIVES

Many overhead camshaft engines use a timing belt ratherthan a chain. Cam-driven belts are made from rubberand fabric and are usually reinforced with fiberglass orKevlar®. The belt sprocket teeth are square-cut or cogged.The belt sprockets are usually made from aluminum. Drivebelts and sprockets reduce weight compared to a chaindrive and require no lubrication with reduced noise. However, the belt requires periodic replacement, usuallyevery 60,000 miles (100,000 kilometers). See Figures 24-19 and 24-20.

Unless the engine is freewheeling, the piston can hit thevalves if the belt breaks. See Figure 24-21.

TENSIONERLEVER

TENSIONER

SECONDARYCHAINS

INTERMEDIATESPROCKET

PRIMARYCHAIN

FIGURE 24-18 Typical dual-overhead camshaft V-type engine that uses one primary timing chain and two secondary chains.

TENSIONER

EXHAUSTCAMSHAFTINTAKE

CAMSHAFTDRIVEBELT

FIGURE 24-19 A typical 4-cylinder double overhead camshaft(DOHC) engine, which uses a belt drive from the camshaft.


446 CHAPTER 24


must have some means to control the shaft end thrust. Twomethods are in common usage. One method is to use a thrustplate between the camshaft drive gear or sprocket and aflange on the camshaft. See Figures 24-22 and 24-23. Thisthrust plate is attached to the engine block with cap screws.In a few camshafts, a button, spring, or retainer that contactsthe timing cover limits forward motion of the camshaft.

LIFTER ROTATION

Valve trains that use flat-bottom lifters use a spherical (curved)lifter face that slides against the cam lobe. This produces asurface on the lifter face that is slightly convex, by about 0.002 inch. The lifter also contacts the lobe at a point that is

FIGURE 24-20 Notice the teeth missing from this timing belt.Thisbelt broke at 88,000 miles because the owner failed to replace it at the recom-mended interval of 60,000 miles.

INTERFERENCEENGINE DESIGN

FREEWHEELINGENGINE DESIGN

NO VALVE/PISTONINTERFERENCE

VALVE/PISTONCOLLISION

FIGURE 24-21 Many engines are of the interference design. If thetiming belt (or chain) breaks, the piston still moves up and down in the cylinderwhile the valves remain stationary.With a freewheeling design,nothing is dam-aged, but in an interference engine, the valves are often bent.

CAMSHAFT MOVEMENT

As the camshaft lobe pushes the lifter upward against the valvespring force, a backward twisting force is developed on thecamshaft. After the lobe goes past its high point, the liftermoves down the backside of the lobe. This makes a forwardtwisting force. This action produces an alternating torsionforce forward, then backward, at each cam lobe. This alternat-ing torsion force is multiplied by the number of cam lobes onthe shaft. The camshaft must have sufficient strength to mini-mize torsion twist and also be tough enough to minimize fatigue from the alternating torsion forces.

Cam chucking is the movement of the camshaftlengthwise in the engine during operation. Each camshaft

BACK END OF CAMSHAFT

THRUST PLATE

FIGURE 24-22 Thrust plate controlling the camshaft end thrust onan overhead camshaft engine.

THRUSTPLATE

FIGURE 24-23 Typical thrust plate between the cam gear and aflange on the camshaft. Note the hole in the fiber composition gear to provideaccess to the thrust plate bolts.




CLEARANCE

NOTE THAT THE LIFTER BORES ARE OFFSET TO ALLOW ROTATION.

c

NOTE: THE TAPER IS USUALLY BETWEEN 0.0007" TO 0.002".

ABOUT 0.002"

THIS CAUSES THIS

MOST LATE MODEL AUTOMOTIVE CAMS ARE TAPERED TO PROVIDE LIFTER ROTATION. THE LIFTERS HAVE A SPHERICAL GRIND SO THAT THEY DO NOT RIDE ON THE EDGE OF THE CAM LOBE. THIS CONTACT SPREADS THE LOAD OF THE VALVE TRAIN AGAINST MORE OF THE LOBE FACE.

POINT CONTACT OF THE CAMLOBES AND THE TAPPET WILLCAUSE SPALLING OF THE LIFTERAND EARLY FAILURE OF BOTH PARTS.

RIGHT

WRONG

FIGURE 24-24 New lifters shouldalways be used with a new camshaft. If wornlifters are used on a new camshaft,edge wear onthe cam lobes will quickly wear the camshaft.

After any engine equipped with roller lifters is run for ashort time, it will wear a path on the camshaft. The pathtraveled by the roller over the cam causes the area to havea mirrorlike appearance. The area on both sides of thisshiny path retains the dull finish of the original camshaft.

This wear pattern is often mistakenly assumed to be abnormal, and as a result, the camshaft and liftersare sometimes needlessly replaced. To avoid replacinggood parts or not replacing worn parts, always carefullymeasure all engine parts.

Tech TipROLLER LIFTER

CAM WEAR

slightly off center. This produces a small turning force on thelifter to cause some lifter rotation for even wear. In operation,there is a wide line of contact between the lifter and the highpoint of the cam lobe. See Figure 24-24.

These are the highest loads that are produced in an en-gine. This surface is the most critical lubrication point in anengine.

CAMSHAFT LIFT

The lift of the cam is usually expressed in decimal inches andrepresents the distance that the valve is lifted off the valveseat. See Figures 24-25 and 24-26.

The higher the lift, the more air and fuel that can theo-retically enter the engine. The more air and fuel burned inan engine, the greater the power potential of the engine.The amount of lift of a camshaft is often different for the


448 CHAPTER 24


higher ratio are installed (for example, 1.6:1 rockers replacingthe stock 1.5:1 rocker arms), the lift at the valve is increased.Also, because the rocker arm rotation covers a greater distanceat the pivot of the rocker arm, the rocker arm can hit the edgeof the valve retainer.

ROCKER ARMS

A rocker arm reverses the upward movement of the pushrodto produce a downward movement on the tip of the valve.Engine designers make good use of the rocker arm. It is de-signed to reduce the travel of the cam follower or lifter andpushrod while maintaining the required valve lift. This isdone by using a rocker arm ratio of 1.5:1, as shown inFigure 24-27.

For a given amount of lift on the pushrod, the valve willopen to 1.5 times the pushrod lift distance. This ratio allowsthe camshaft to be small, so the engine can be smaller. It alsoresults in lower lobe-to-lifter rubbing speeds.

A technician replaced a timing chain and gears on aChevrolet V-8. The repair was accomplished correctly, yetafter starting, the engine burned an excessive amount ofoil. Before the timing chain replacement, oil consumptionwas minimal. The replacement timing chain restoredproper operation of the engine and increased engine vac-uum. Increased vacuum can draw oil from the crankcasepast worn piston rings and through worn valve guides dur-ing the intake stroke. Similar increased oil consumptionproblems occur if a valve regrind is performed on a high-mileage engine with worn piston rings and/or cylinders.

To satisfy the owner of the vehicle, the technicianhad to disassemble and refinish the cylinders and replacethe piston rings. Therefore, all technicians should warncustomers that increased oil usage may result from almostany repair to a high-mileage engine.

TechTipBEST TO WARN

THE CUSTOMER

A

B

A - B = CAM LIFT

B

A - B DOES NOTEQUAL CAM LIFT

A

FIGURE 24-26 Some cam lobes provide lift for more than 180 de-grees of camshaft rotation, so it is not possible to get an accurate base circlemeasurement using a micrometer.

intake and exhaust valves. If the specifications vary, thecamshaft is called asymmetrical. If the lift is the same, thecam is called symmetrical. However, when the amount oflift increases, so do the forces on the camshaft and the restof the valve train. Generally, a camshaft with a lift of over0.500 inch (1.3 centimeters) is unsuitable for street opera-tion except for use in engines that are over 400 cubic inches(6.0 liters).

The lift specifications at the valve face assume the use ofthe stock rocker arm ratio. If nonstock rocker arms with a

LOBE LIFT

FIGURE 24-25 Lobe lift is the amount the cam lobe lifts the lifter.




Rocker arms may be cast, forged, or stamped. Forged rockerarms are the strongest, but they require expensive manufac-turing operations. Rocker arms may have bushings or bearingsinstalled to reduce friction and increase durability. Cast rockerarms cost less to make and do not usually use bushings, butthey do require several machining operations. They are not asstrong as forged rocker arms but are satisfactory for passengervehicle service.

Shaft-Mounted Rocker Arms

On some overhead valve and most single overhead camshaftengines, the rocker arms are mounted on a shaft that runs the full length of the cylinder head. See Figures 24-29and 24-30.

Because the shaft provides a strong and stable platform forthe rocker arms, shaft-mounted rocker arms work well, espe-cially at high engine speeds. While shaft-mounted rocker armsresist flex and accurately transmit the cam profile to the valve,they do add weight and cost to the engine.

While most overhead camshaft engines and some over-head valve (OHV) engines that use rocker arm shafts do usean adjustable rocker arm, most OHV engines have no provi-sion for adjustment. Shaft-mounted rocker arms are lubri-cated through oil passages that travel from the blockthrough the head and into the shaft, and then to the rockerarms.

THE NOISY CAMSHAFT

The owner of an overhead cam 4-cylinder engine com-plained of a noisy engine. After taking the vehicle to sev-eral technicians and getting high estimates to replacethe camshaft and followers, the owner tried to find aless expensive solution. Finally, another technician re-placed the serpentine drive belt on the front of the en-gine and “cured” the “camshaft” noise for a fraction ofthe previous estimates.

Remember, accessory drive belts can often makenoises similar to valve or bad bearing types of noises.Many engines have been disassembled and/or over-hauled because of a noise that was later determined tobe from one of the following:■ Loose or defective accessory drive belt(s)■ Loose torque converter-to-flex plate (drive plate)

bolts (nuts)■ Defective mechanical fuel pump

Real WorldFix

VALVE

.350”

LIFTER

CAMLOBE

PUSHROD

.525”

PIVOT POINTOR SHAFT

ROCKER ARM

1”1.5”

FIGURE 24-27 A rocker arm showing a 1.5:1 ratio. The distancefrom the pivot to the pushrod seat is shorter than the distance from the pivot to the valve stem.

CAUTION: Using rocker arms with a higher ratio than stock can also cause

the valve spring to compress too much and actually bind. Valve spring bind

(coil bind) occurs when the valve spring is compressed to the point where

there is no clearance in the spring. (It is completely compressed.) When coil

bind occurs in a running engine, bent pushrods, broken rocker arms, or other

valve train damage can result. See Figure 24-28.

VALVESPRING

FIGURE 24-28 If the space between the coils is not adequate, coilbind occurs.

ROCKER ARMSHAFT

FIGURE 24-29 Typical rocker arm shaft design on an overheadvalve engine.


450 CHAPTER 24


Stud-Mounted Rocker Arms

Stud-mounted rockers are only found on overhead valve(OHV) engines and each rocker arm is attached to a stud thatis pressed or threaded into the cylinder head. A ball on top ofthe rocker arm provides the bearing surface as the rocker armpivots and is held in place and adjusted for valve clearance bya nut. While this design looks less stable than a shaft-mountedrocker, this design has proved to be reliable and is inexpensiveto manufacture. Some engines use pushrod guide plates fas-tened to the head. See Figures 24-31 and 24-32. The rockerarms are lubricated through hollow pushrods.

Pedestal-Mounted Rocker Arms

Pedestal-mounted rocker arms are similar to stud-mountedrocker arms but do not use a stud and are used only in over-head valve engines. Two rocker arms are attached to and pivoton a pedestal attached to the cylinder head with one or twobolts. The rocker arms are usually stamped steel, which islightweight and not adjustable. See Figures 24-33 and 24-34.The rocker arms are lubricated through hollow pushrods.

ADJUSTMENTSCREW

EXHAUSTVALVE

VALVESPRING

LOCKNUT

CYLINDERHEADINTAKEVALVE

CAMSHAFT

ROCKERSHAFT

ROCKERARM

FIGURE 24-30 A typical single overhead camshaft (SOHC) enginewith shaft-mounted rocker arms, which ride directly on the camshaft.

FIGURE 24-31 Pushrod guide plates are bolted to the head andhelp stabilize the valve train, especially at high engine speeds.

STUD

VIEW “A”

A

ROCKERARM

FIGURE 24-32 A typical stud-mounted rocker arm.

BODY

VALVESPRING

COLLAR

COILSPRING

VALVE ROTATOR

FLATWASHER

INTAKEVALVE

EXHAUSTVALVE

PUSHRODS

ROCKERARMS

ROCKERARMPIVOT

EXHAUSTVALVESEAL

VALVEKEEPERS

VALVEROTATORS

INTAKEVALVESEAL

SPRINGS

FIGURE 24-33 A pedestal-type rocker arm design that uses twobolts for each rocker arm pivot and is not adjustable.

PUSHRODS

Pushrods transfer the lifting motion of the valve train from thecam lobe and lifters to the rocker arms. See Figure 24-35.

Pushrods are designed to be as light as possible and stillmaintain their strength. They may be either solid or hollow. Ifthey are to be used as passages for oil to lubricate rocker arms,they must be hollow. Pushrods use a convex ball on the lowerend that seats in the lifter. The rocker arm end is also a con-vex ball, unless there is an adjustment screw in the pushrodend of the rocker arm. In this case, the rocker arm end of thepushrod has a concave socket. It mates with the convex ballon the adjustment screw in the rocker arm. Pushrod end typesare shown in Figure 24-36.

All pushrods should be rolled on a flat surface to check forstraightness. See Figure 24-37.




The tolerance in the valve train allows for some machiningof engine parts without the need to change pushrod length.However, if one or more of the following changes have beenmade to an engine, a different pushrod length may be necessary:

■ Block deck height machined■ Cylinder head deck height machined■ Camshaft base circle size reduced■ Valve length increased■ Lifter design changed

BOLT

OILDEFLECTOR

PEDESTAL

ROCKERARM

FIGURE 24-34 A pedestal-type rocker arm design that used onebolt for each rocker arm and is not adjustable. If valve lash needs to be adjusted,different length pushrod(s) must be used.

As oil oxidizes, it forms a varnish. Varnish buildup is par-ticularly common on hot upper portions of the engine,such as rocker arm shafts. The varnish restricts clean oilfrom getting into and lubricating the rocker arms. Thecam lobe can easily force the valves open, but the valvesprings often do not exert enough force to fully close the valves. The result is an engine miss, which may beintermittent. Worn valve guides and/or weak valvesprings can also cause occasional rough idle, unevenrunning, or missing.

TechTipROCKER ARM

SHAFTS CAN CAUSE

STICKING VALVES

CAMSHAFT

LIFTER

ROCKERARM

PUSHROD

FIGURE 24-35 Overhead valve engines are also known as push-rod engines because of the long pushrod that extends from the lifter to therocker arm.

FIGURE 24-36 Types of pushrod ends.

FIGURE 24-37 It was easy to see that these pushrods needed to bereplaced because they became bent when the timing chain broke.


452 CHAPTER 24


Many engine rebuilders and remanufacturers do notreuse old hollow pushrods. Dirt, carbon, and other debrisare difficult to thoroughly clean from inside a hollowpushrod. When an engine is run with used pushrods,the trapped particles can be dislodged and ruin newbearings and other new engine parts.

TechTipHOLLOW

PUSHROD DIRT

All pushrods used with guide plates must be hardenedon the sides and on the tips. To easily determine if apushrod is hardened, simply use a sharp pocketknife toscrape the wall of the pushrod. A heat-treated pushrodwill not scratch. See Figure 24-38.

TechTipTHE SCRATCH TEST

FIGURE 24-38 Hardened pushrods should be used in any enginethat uses pushrod guides (plates). To determine if the pushrod is hardened,simply try to scratch the side of the pushrod with a pocketknife.

OVERHEAD CAMSHAFT

VALVE TRAINS

Overhead camshaft engines use several methods for openingthe valves.

1. One type opens the valves directly with a bucket. SeeFigure 24-39.

2. The second type uses a cam follower, also called a fingerfollower, that provides an opening ratio similar to that ofa rocker arm. See Figure 24-40. Finger followers open thevalves by approximately 1 1/2 times the cam lift. The pivotpoint of the finger follower may have a mechanical adjust-ment or it may have an automatic hydraulic adjustment.

3. A third type moves the rocker arm directly through ahydraulic lifter. See Figure 24-41.

4. In the fourth design, some newer engines have thehydraulic adjustment in the rocker arm and arecommonly called hydraulic lash adjusters (HLA).See Figure 24-42.

LIFTER

VALVE STEMCONTACTS HERE

OILENTERSHERE

CAMSHAFT

HYDRAULICADJUSTER

FIGURE 24-39 Hydraulic lifters may be built into bucket-typelifters on some overhead camshaft engines.

CAMSHAFT HYDRAULICLIFTER

CAMFOLLOWER

FIGURE 24-40 The use of cam followers allow the use of hydrauliclifters with an overhead camshaft design.




FIGURE 24-41 The overhead cam operates the rocker arm througha hydraulic lifter.

CAMSHAFT

HYDRAULICLIFTER

ROCKERARM

FIGURE 24-42 Hydraulic lifters (hydraulic lash adjusters) may bebuilt into rocker arms on some overhead camshaft engines.

CAMSHAFT

SPECIFICATIONS

Camshaft duration is the number of degrees of crankshaft ro-tation for which the valve is lifted off the seat. The specifica-tions for duration can be different for the intake valves and theexhaust valves. If the durations of the intake and exhaust valvesare different from each other, the cam is called asymmetrical.The specification for duration can be expressed by several dif-ferent methods, which must be considered when comparingone cam with another. The three most commonly used meth-ods are as follows:

1. Duration of valve opening at zero lash (clearance).If a hydraulic lifter is used, the lash is zero. If a solid lifteris used, this method of expression refers to the durationof the opening of the valve after the specified clearance(lash) has been closed.

2. Duration at 0.050-inch lifter (tappet) lift. Becausethis specification method eliminates all valve lash clear-ances and compensates for lifter (tappet) styles, it is thepreferred method to use when comparing one camshaftwith another. Another method used to specify durationof some factory camshafts is to specify crankshaft dura-tion at 0.010-inch lifter lift. The important point to re-member is that the technician must be sure to use

equivalent specification methods when comparing or se-lecting camshafts.

NOTE: Fractions of a degree are commonly expressed inunits called minutes ('). Sixty minutes equal onedegree. For example, 45' � 3/4 degree, 30' � 1/2degree, and 15' � 1/4 degree.

3. SAE camshaft specifications. The valve timing andvalve overlap are expressed in the number of degrees ofcrankshaft rotation for which the valves are off their seats.SAE’s recommended practice is to measure all valveevents at 0.006-inch (0.15-millimeter) valve lift. Thismethod differs from the usual method used by vehicle orcamshaft manufacturers. Whenever comparing valve tim-ing events, be certain that the exact same methods areused on all camshafts being compared.


454 CHAPTER 24


Valve Overlap

Another camshaft specification is the number of degrees ofoverlap. Camshaft overlap is the number of degrees of crank-shaft rotation between the exhaust and intake strokes forwhich both valves are off their seats.

■ A lower amount of overlap results in smoother idle andlow-engine speed operation, but it also means that a loweramount of power is available at higher engine speeds.

■ A greater valve overlap causes rougher engine idle, withdecreased power at low speeds, but it also means thathigh-speed power is improved.

For example: A camshaft with 50 degrees (or less) of over-lap may be used in an engine in which low-speed torque andsmooth idle qualities are desired. Engines used with overdriveautomatic transmissions benefit from the low-speed torqueand fuel economy benefits of a small-overlap cam. A camshaftwith 100 degrees of overlap is more suitable for use with amanual transmission, with which high-RPM power is desired.An engine equipped with a camshaft with over 100 degrees ofoverlap tends to idle roughly and exhibit poorer low-enginespeed response and lowered fuel economy. See Figure 24-43.

The valve overlap is the number of degrees for which bothvalves are open near TDC. In the previous example, the intakevalve starts to open at 19 degrees. The exhaust valve is alsoopen during this upward movement of the piston on the ex-haust stroke. The exhaust valve is open until 22 degrees ATDC.

To determine overlap, total the number of degrees forwhich the intake valve is open BTDC (19 degrees) and thenumber of degrees for which the exhaust valve is open ATDC(22 degrees):

Valve overlap � 19° � 22° � 41°

Lobe Centers

Another camshaft specification that creates some confusion isthe angle of the centerlines of the intake and exhaust lobes.This separation between the centerlines of the intake and

TDC

EXHAUSTOPEN STARTING

TO CLOSE

INTAKE VALVEOPENING

TDCBDCTDCBDC

EXHAUST VALVESTARTING TO OPEN EXHAUST

INTAKE VALVEOPENINGINTAKE

OVERLAP

720°540°360°180°0°

CRANKSHAFT ROTATION

FIGURE 24-43 Graphic representa-tion of a typical camshaft showing the relationshipbetween the intake and exhaust valves.

exhaust lobes is called lobe center, lobe separation, lobedisplacement angle (LDA), or lobe spread and is mea-sured in degrees. See Figure 24-44.

Two camshafts with identical lift and duration can varygreatly in operation because of variation in the angle betweenthe lobe centerlines.

1. The smaller the angle between the lobe centerlines, thegreater the amount of overlap. For example, 108 degreesis a narrower lobe center angle.

2. The larger the angle between the lobe centerlines, theless the amount of overlap. For example, 114 degrees isa wider lobe center angle.

NOTE: Some engines that are equipped with dual overhead camshafts

and four valves per cylinder use a different camshaft profile for each of the

intake and exhaust valves. For example, one intake valve for each cylinder

could have a cam profile designed for maximum low-speed torque.The other

intake valve for each cylinder could be designed for higher-engine-speed

power. This results in an engine that is able to produce a high torque over a

broad engine speed range.

To find the degree of separation between intake andexhaust lobes of a cam, use the following formula:

(Intake duration � Exhaust duration)/4 � Overlap/2 �Number of degrees of separation

OVERLAP112°

EXHAUSTLOBE

INTAKELOBE

FIGURE 24-44 As the lobe center angle decreases, the overlapincreases, with no other changes in the lobe profile lift and duration.




Typical intake valve specifications are to open at 19 degreesbefore top dead center (BTDC) and close at 46 degrees afterbottom dead center (ABDC).

Exhaust Valve

The exhaust valve opens while the piston is traveling downon the power stroke, before the piston starts up on the ex-haust stroke. Opening the exhaust valve before the pistonstarts up on the exhaust stroke ensures that the combustionpressure is released and the exhaust valve is mostly openwhen the piston does start up. The exhaust valve does notclose until after the piston has traveled past TDC and is start-ing down on the intake stroke. Because of inertia of theexhaust, some of the burned gases continue to flow out theexhaust valve after the piston is past TDC. This can leave apartial vacuum in the combustion chamber to start pulling inthe fresh charge.

Typical exhaust valve specifications are to open at 49 de-grees before bottom dead center (BBDC) and close at 22 degreesafter top dead center (ATDC).

Cam Timing Chart

During the four strokes of a four-stroke-cycle gasoline engine,the crankshaft revolves 720 degrees (it makes two completerevolutions [2 � 360° � 720°]). Camshaft specifications aregiven in crankshaft degrees. In the example in Figure 24-46,the intake valve starts to open at 39 degrees BTDC, remainsopen through the entire 180 degrees of the intake stroke, anddoes not close until 71 degrees ATDC. Therefore, the durationof the intake valve is 39 degrees � 180 degrees � 71 degrees,or 290 degrees.

The exhaust valve of the example camshaft opens at 78degrees BBDC and closes at 47 degrees ATDC. When the ex-haust valve specifications are added to the intake valve spec-ifications in the diagram, the overlap period is easily observed.The overlap in the example is 39 degrees � 47 degrees, or

See Figure 24-45 for a typical camshaft valve timing diagram.

The lobe separation angle can be determined by trans-ferring the intake and exhaust duration and overlap into theformula as follows:

Intake duration � 15° � 59° � 180° � 254°

Exhaust duration � 59° � 15° � 180° � 254°

Overlap � 15° � 15° � 30°

(254° � 254°)/4 � 30°/2 � 508°/4 � 30°/2 � 127° � 15° � 112°

CAM TIMING

SPECIFICATIONS

Cam timing specifications are stated in terms of the angle ofthe crankshaft in relation to top dead center (TDC) or bottomdead center (BDC) when the valves open and close.

Intake Valve

The intake valves should open slightly before the pistonreaches TDC and starts down on the intake stroke. This en-sures that the valve is fully open when the piston travelsdownward on the intake stroke. The flow through a partiallyopen valve (especially a valve ground at 45 degrees instead of30 degrees) is greatly reduced as compared with that whenthe valve is in its fully open position. The intake valve closesafter the piston reaches BDC because the air–fuel mixture hasinertia, or the tendency of matter to remain in motion. Evenafter the piston stops traveling downward on the intake strokeand starts upward on the compression stroke, the inertia of theair–fuel mixture can still be used to draw in additional charge.

INTAKE CLOSES EXHAUST OPENS59°59°

INTAKE OPENS EXHAUST CLOSES

15°15°

FIGURE 24-45 Typical cam timing diagram.

TDC

POWER

180°

EXHAUST

180°

INTAKE

180°

COMPRESSION

180°

TDCBDCTDCBDC

71°47°39°78°

OVERLAP = 86°

FIGURE 24-46 Typical high-performance camshaft specificationson a straight-line graph. Intake valve duration � 39° � 180° � 71° � 290°.Exhaust valve duration � 78° � 180° � 47° � 305°. Because intake and exhaust valve specifications are different, the camshaft grind is called asymmetrical.


456 CHAPTER 24


86 degrees. The duration of the exhaust valve opening is 78 degrees � 180 degrees � 47 degrees, or 305 degrees. Because the specifications of this camshaft indicate closeto and over 300 degrees of duration, this camshaft shouldonly be used where power is more important than fuel economy.

The usual method of drawing a camshaft timing diagramis in a circle illustrating two revolutions (720 degrees) of thecrankshaft. See Figure 24-47 for an example of a typicalcamshaft timing diagram for a camshaft with the same specifi-cations as the one illustrated in Figure 24-46.

MEASURING AND

REGRINDING CAMSHAFTS

All camshafts should be checked for straightness by placingthem on a V block and measuring the cam bearings for runoutby using a dial indicator. The maximum total indicatorrunout (TIR) should be less than 0.002 inch (0.05 milli-meter). See Figure 24-48.

Worn camshafts can be restored to original lift and dura-tion by one of two methods:

1. If the camshaft is not excessively worn (less than 0.030inch), the lobes can be reground by decreasing the diam-eter of the base circle, restoring the original lift and du-ration. See Figures 24-49 and 24-50.

2. If the cam lobe wear is excessive, the lobes can be weldedand reground back to their original specifications.

NOTE: According to major engine remanufacturers, only about 35% of

camshafts can be reground. Therefore, about two-thirds of the camshafts

received in engine cores are excessively worn and must be replaced.

ROTATION

EXHAUST CLOSES47°

INTAKE OPENS39°

ROTATION

78°EXHAUST OPENS

71°INTAKE CLOSES

BOTTOM DEAD CENTER

THIS VALVE TIMING DIAGRAM SHOWS TWOREVOLUTIONS (720°) OF THE CRANKSHAFT

FIGURE 24-47 Typical camshaft valve timing diagram with thesame specifications as those shown in Figure 24-46.

DIAL INDICATOR

CAM BEARINGJOURNAL

FIGURE 24-48 A camshaft being checked for total indicator runoutas it is being rotated on V blocks using a dial indicator.

INSTALLING THE

CAMSHAFT

When the camshaft is installed, the lobes must be coated witha special lubricant that contains molydisulfide. This speciallube helps to ensure proper initial lubrication to the criticalcam lobe sections of the camshaft. Many manufacturers rec-ommend multiviscosity engine oil such as SAE 5W-30 or SAE10W-30. Some camshaft manufacturers recommend usingstraight SAE 30 or SAE 40 engine oil and not a multiviscosityoil for the first oil fill. Some manufacturers also recommendthe use of an antiwear additive such as zinc dithiophosphate(ZDP). See Figure 24-51.

NOTE: Most camshafts are coated at the factory with a polycrystalline-

structure chemical treatment. This coating is typically manganese phos-

phate and gives the camshaft a dull black appearance. The purpose of this

treatment is to absorb and hold oil to help ensure lubrication during the

break-in period. Under a microscope, this surface treatment looks like the

surface of a golf ball.




A flat-bottom lifter camshaft must be broken in bymaintaining engine speed above 1500 RPM for the first 10 minutes of engine operation. If the engine speed is de-creased to idle (about 600 RPM), the lifter (tappet) will bein contact with and exerting force on the lobe of the camfor a longer period of time than occurs at higher enginespeeds. The pressure and volume of oil supplied to thecamshaft area are also increased at the higher enginespeeds. Therefore, to ensure long camshaft and lifter life,make certain that the engine will start quickly after re-assembly to prevent long cranking periods and subsequentlow engine speeds after a new camshaft and lifters have

been installed. Whenever repairing an engine, follow theserules regarding the camshaft and lifters:

NOTE: When installing a roller lifter camshaft, no break-in is necessary.

1. When installing a new camshaft, always install newvalve lifters (tappets).

ORIGINAL CAM

WORN CAM

LOSS OF LIFT 0.020

WORN LIFT 0.380

LIFT 0.400

B 0.980 0.600”

B 1.000

0.600”

A (0.600) A (0.600)

B – LOSS OF LIFT (0.020) = 0.980 B (0.980) – A (0.600) = 0.380 0.380 WORN LIFT

B – A = LIFT B (1.000) – A (0.600) = 0.400 LIFT

B (1.000) – (2 � 0.020) = 0.960A (0.600) – (2 � 0.020) = 0.560B (0.960) – A (0.560) = 0.4000.400 RESTORED LIFT

0.020

A (0.600)

A (0.560) 0.020

B 0.960

B 1.000

0.560”

RESTORED LIFT 0.400

REGROUND CAM 0.020

FIGURE 24-49 A worn camshaft lobe can be restored to its original lift by grinding the base circle smaller.

CAMSHAFT

CAMSHAFTGRINDING STONE

FIGURE 24-50 A camshaft being ground on a camshaft grindingmachine.

FIGURE 24-51 Special lubricant such as this one from General Motors is required to be used on the lobes of the camshaft and the bottom of the flat-bottomed lifters.


458 CHAPTER 24


2. When installing new lifters, if the original cam is notexcessively worn and if the pushrods all rotate with theoriginal camshaft, the camshaft may be reused.

NOTE: Some manufacturers recommend that a newcamshaft always be installed when replacing valvelifters.

3. Never use a hydraulic camshaft with solid lifters or hydraulic lifters with a solid lifter camshaft.

4. New flat-bottom lifters will be more compatible if thebottom part of the lifter that contacts the cam is polishedwith #600 grit sandpaper.

NOTE: Many molydisulfide greases can start to clog oil filters within

20 minutes after starting the engine. Most engine rebuilders recommend

changing the oil and filter after one-half hour of running time.

Common Usage Seat Duration LiftDuration at 0.050 Inch Characteristics

Street 246°–254° 0.400 192°–199° Smooth idle, power idle to 4500 RPMStreet 262° 0.432 207° Broad power range, smooth idle, power idle to 4800 RPMStreet 266° 0.441 211° Good idle for 350-cubic-inch engines, power idle to 5200 RPMStreet/drag strip 272° 0.454 217° Lope idle, power idle to 5500 RPMStreet/racetrack 290° 0.500 239° Shaky idle, power idle to 5500–6500 RPM

A common mistake of beginning engine builders is to in-stall a camshaft with too much duration for the size ofthe engine. This extended duration of valve opening re-sults in a rough idle and low manifold vacuum, whichcauses carburetor metering problems and lack of low-speed power.

For example, a hydraulic cam with a durationgreater than 225 degrees at 0.050-inch lift for a 350-cubic-inch engine will usually not be suitablefor street driving. Seat duration is the number of degrees of crankshaft rotation that the valve is off theseat.

TechTip

TOO BIG � TOO BAD

DEGREEING THE

CAMSHAFT

The purpose of degreeing the camshaft in the engine is to locate the valve action exactly as the camshaft manufacturersintended. The method most often recommended by camshaftmanufacturers is the intake lobe centerline method. Thismethod determines the exact centerline of the intake lobe andcompares it to the specifications supplied with the replacementcamshaft. On an overhead valve engine, the camshaft is usuallydegreed after the crankshaft, piston, and camshaft are installedand before the cylinder heads are installed. To determine thecenterline of the intake lobe, follow these steps using a degreewheel mounted on the crankshaft:

Step 1 Locate the exact top dead center. Install a degreewheel and bring the cylinder #1 piston close toTDC. Install a piston stop. (A piston stop is any ob-ject attached to the block that can act as a solid me-chanical stop to prevent the piston from reachingthe top of the cylinder.) Turn the engine clockwiseuntil the piston gently hits the stop.

CAUTION: Do not use the starter motor to rotatethe engine. Use a special wrench on the flywheel orthe front of the crankshaft.

Record the reading on the degree wheel, and thenturn the engine in the opposite direction until it stopsagain and record that number. Figure 24-52 indicatesa reading of 30 degrees ATDC and 26 degrees BTDC.Add the two readings together and divide by two(30° � 26° � 56° � 2 � 28°). Move the degree

3530

25 20 15 10 5 10 15 20 2530

35

5

ATDC BTDC

TDC

FIGURE 24-52 Degree wheel indicating where the piston stoppednear top dead center. By splitting the difference between the two readings, thetrue TDC (28 degrees) can be located on the degree wheel.




wheel until it is 28 degrees and the engine hasstopped rotating in either direction. Now TDC onthe degree wheel is exactly at top dead center.

Step 2 Remove the piston stop and place a dial indicatoron an intake valve lifter. To accurately locate thepoint of maximum lift (intake lobe centerline), ro-tate the engine until the lifter drops 0.050 inch on

each side of the maximum lift point. Mark the degreewheel at these points on either side of the maximumlift point. Now count the degrees between these twopoints and mark the halfway point. This halfwaypoint represents the intake centerline. This pointis often located between 100 degrees and 110 degrees. See Figure 24-53.

(a)

FIGURE 24-53 (a) Photo showing the setup required to degree acamshaft. (b) Close-up of the pointer and the degree wheel. (c) The dial indica-tor used to find exact top dead center.

(b)

DEGREE WHEEL

(c)


460 CHAPTER 24


Step 3 Now that both TDC and intake centerline havebeen marked, compare the actual intake centerlinewith the specification. For example, if the actualintake centerline is 106 degrees and the camshaftspecification indicates 106 degrees, then thecamshaft is installed straight up. See Figure 24-54.If the actual reading is 104 degrees, the camshaftis advanced by 2 degrees. If the actual reading is 108 degrees, the camshaft is retarded by 2 degrees.

Advanced Cam Timing

If the camshaft is slightly ahead of the crankshaft, the camshaftis called advanced. An advanced camshaft (maximum of4 degrees) results in more low-speed torque with a slight

decrease in high-speed power. Some aftermarket camshaftmanufacturers design about a 4-degree advance into their tim-ing gears or camshaft. This permits the use of a camshaft withmore lift and duration, yet still provides the smooth idle andlow-speed responses of a milder camshaft.

Retarded Cam Timing

If the camshaft is slightly behind the crankshaft, the camshaftis called retarded. A retarded camshaft (maximum of 4 degrees)results in more high-speed power at the expense of low-speedtorque.

If the measured values are different from specifications,special offset pins or keys are available to relocate the cam gearby the proper amount. Some manufacturers can provide adjustable cam timing sprockets for overhead cam engines.

INTAKE

OVERLAP

106°

TOP DEAD CENTER(TDC)

EXHAUST LOBECENTERLINE

INTAKE LOBECENTERLINE

BOTTOM DEAD CENTER(BDC)

EXHAUST

CLOSES

EXHAUST

EXHAUST

OPENS

INTAKE

OPENS

INTAKE CLOSES

316°

99°

47°52°

106°

306°

84°79°

DURATION

CYLINDER UNDER PRESSURE 197°

EXHAUST DURATION

FIGURE 24-54 Typical valve timing diagram showing the intake lobe centerline at 106 degrees ATDC.




VARIABLE VALVE TIMING

Conventional camshafts are permanently synchronized to thecrankshaft so that they operate the valves at a specific point ineach combustion cycle. In an engine, the intake valve opensslightly before the piston reaches the top of the cylinder andcloses about 60 degrees after the piston reaches the bottom ofthe stroke on every cycle, regardless of the engine speed or load.

Variable-cam timing allows the valves to be operated at different points in the combustion cycle, to improve performance. See the chart.

There are three basic types of variable valve timing usedon General Motors vehicles:

1. Exhaust camshaft variable action only on overheadcamshaft engines, such as the inline 4.2-liter 4200 usedin Trailblazers.

2. Intake and exhaust camshaft variable action on bothcamshafts used in many General Motors engines.

3. Overhead valve, cam-in-block engines use variable valvetiming by changing the relationship of the camshaft tothe crankshaft.

If the cam timing is advanced (relative to the crank-shaft), the intake valve-to-piston clearance is reduced. Ifthe cam timing is retarded, the exhaust valve-to-pistonclearance is reduced.

This is true because the intake valve lags behind themotion of the piston on the intake stroke, whereas thepiston “chases” the exhaust valve on the exhaust stroke.See Figure 24-55.

TechTipVALVE-TO-PISTON

CLEARANCE

VERSUS CAM

TIMING

FIGURE 24-55 Modeling clay was used to determine valve-to-piston clearance. Most manufacturers recommend a minimum of 0.070 inch (1.8 millimeters). The clay is cut with a knife and the thickness of the clayis measured to determine the static (engine not running) clearance. The clear-ance decreases as the speed of the engine increases because of valve timingvariations connecting rod stretch.

Driving ConditionChange inCamshaft Position Objective Result

Idle No change Minimize valve overlap Stabilize idle speedLight engine load Retard valve timing Decrease valve overlap Stable engine outputMedium engine load Advance valve timing Increase valve overlap Better fuel economy with lower emissionsLow to medium RPM Advance valve timing Advance intake Improve low to midrange torquewith heavy load valve closingHigh RPM with Retard valve timing Retard intake valve closing Improve engine outputheavy load

The camshaft position actuator oil control valve (OCV)directs oil from the oil feed in the head to the appropriatecamshaft position actuator oil passages. There is one OCV foreach camshaft position actuator. The OCV is sealed andmounted to the front cover. The ported end of the OCV is in-serted into the cylinder head with a sliding fit. A filter screen pro-tects each OCV oil port from any contamination in the oil supply.

The camshaft position actuator is mounted to the frontend of the camshaft and the timing notch in the nose of thecamshaft aligns with the dowel pin in the camshaft positionactuator to ensure proper cam timing and camshaft positionactuator oil hole alignment. See Figure 24-56.

OHV Variable Timing

The 3900 is the first GM overhead valve (OHV) engine to uti-lize variable valve timing (VVT) and active fuel management(displacement on demand—DOD). Engine size was increased


462 CHAPTER 24

400 cc because the larger displacement was needed to obtaingood performance in the three-cylinder mode.

The variable valve timing system uses electronically con-trolled, hydraulic gear-driven cam phaser that can alter the re-lationship of the camshaft from 15 degrees retard to 25 degreesadvance (40 degrees overall) relative to the crankshaft. By usingvariable valve timing (VVT), GM engineers were able toeliminate the EGR valve. The VVT also works in conjunctionwith an active manifold that gives the engine a broader torquecurve.

A valve in the intake manifold creates a longer path for in-take air at low speeds, improving combustion efficiency andtorque output. At higher speed the valve opens creating ashorter air path for maximum power production.

The LZ4 3500 is an OHV engine based on the 3.9 L V-6.It was introduced for the 2006 model year in the ChevroletImpala and Monte Carlo. It includes continuously variablecam timing (fixed overlap). See Figure 24-57.


PADDLE

SPROCKETPADDLECAVITY

SPROCKET

CAMSHAFT

A

A

SECTION A-AFIGURE 24-56 Camshaft ro-tation during advance and retard.

MAGNETICALLY ACTIVATEDOIL CONTROL VALVE

CAMSHAFT PHASER(VANE TYPE)

ELECTROMAGNET

RETURN SPRING

DRIVESPROCKET FRONT

ENGINE COVER

FIGURE 24-57 The camshaft is rotated in relation to the camshaft by the PCM to provide changes in valve timing.

Varying the exhaust and/or the intake camshaft positionallows for reduced exhaust emissions and improved perfor-mance. See the chart.

Camshaft Phasing Changed ImprovesExhaust cam phasing Reduces exhaust emissionsExhaust cam phasing Increases fuel economy (reduced

pumping losses)Intake cam phasing Increases low-speed torqueIntake cam phasing Increases high-speed power

By varying the exhaust cam phasing, vehicle manufacturersare able to meet newer NOX reduction standards and eliminatethe exhaust gas recirculation (EGR) valve. By using exhaust camphasing, the PCM can close the exhaust valves sooner thanusual, thereby trapping some exhaust gases in the combustion




chamber. General Motors uses one or two actuators that allowthe camshaft piston to change by up to 50 degrees in relationto the crankshaft position.

There are two types of cam phasing devices used on General Motors engines:

■ Spline phaser—used on overhead camshaft (OHC) engines■ Vane phaser—used on overhead camshaft (OHC) and

overhead valve (OHV) cam-in-block engines

Spline Phaser System

The spline phaser system is also called the exhaust valve camphaser (EVCP) and consists of the following components:

■ Engine control module (ECM)■ Four-way pulse-width-modulated (PWM) control valve■ Cam phaser assembly■ Camshaft position (CMP) sensor

See Figure 24-58.

Spline Phaser System Operation

On the 4200 inline 6-cylinder engine used in the ChevroletTrailblazer, the pulse-width-modulated (PWM) control valveis located on the front passenger side of the cylinder head. Oil

ADVANCEPOSITION

RETARDPOSITION

OILAPPLIED

OILAPPLIED

DRIVE SPROCKET(FROM CRANKSHAFT)

FLOATINGPISTON

EXHAUSTCAMSHAFT

HELICALSPLINES

STRAIGHT-CUTSPLINES

RELECTOR

FIGURE 24-58 Splinecam phaser assembly.

pressure is regulated by the control valve and then directed tothe ports in the cylinder head leading to the camshaft and camphaser position. The cam phaser is located on the exhaustcams and is part of the exhaust cam sprocket. When the ECMcommands an increase in oil pressure, the piston is moved in-side the cam phaser and rides along the helical splines, whichcompresses the coil spring. This movement causes the camphaser gear and the camshaft to move in an opposite direc-tion, thereby retarding the cam timing. See Figure 24-59.

If a NOX emission failure at a state inspection occurs ora diagnostic trouble code is set related to the cam tim-ing, remove the control valve and check for a cloggedoil screen. A lack of regular oil changes can cause thescreen to become clogged, thereby preventing properoperation. A rough idle is a common complaint becausethe spring may not be able to return the camshaft to theidle position after a long highway trip.

TechTipCHECK THE SCREEN

ON THE CONTROL

VALVE IF THERE

ARE PROBLEMS


464 CHAPTER 24


ENGINE OILPRESSURE

PWM CONTROLVALVE

CAMSHAFT

SPROCKET

SPRINGPISTON

HELICAL SPLINE(PART OF CAM)

RELUCTOR WHEELTOOTH

VENT VENT

CAMSHAFT POSITION SENSOR (CMP)

POWERTRAINCONTROL MODULE (PCM)

CRANKSHAFT POSITION SENSOR (CKP) RPM

MAP SENSOR

FIGURE 24-59 A spline phaser.

NOTE: A unique cam-within-a-cam is used on the 2008�Viper V-10 OHV

engine. This design allows the exhaust lobes to be moved by up to 36° to

improve idle quality and reduction of exhaust emissions.

Vane Phaser System on anOverhead Camshaft Engine

The vane phaser system used on overhead camshaft (OHC)engines uses a camshaft piston (CMP) sensor on eachcamshaft. Each camshaft has its own actuator and its own oilcontrol valve (OCV). Instead of using a piston along a helicalspline, the vane phaser uses a rotor with four vanes, which isconnected to the end of the camshaft. The rotor is located in-side the stator, which is bolted to the cam sprocket. The statorand rotor are not connected. Oil pressure is controlled on bothsides of the vanes of the rotor, which creates a hydraulic link

between the two parts. The oil control valve varies the balanceof pressure on either side of the vanes and thereby controls theposition of the camshaft. A return spring is used under the re-luctor of the phaser to help return it to the home or zero de-grees position. See Figure 24-60.

Magnetically Controlled Vane Phaser

A magnetically controlled vane phaser is controlled by theECM by using a 12-volt pulse-width-modulated (PWM) signalto an electromagnet, which operates the oil control valve(OCV). A magnetically controlled vane phaser is used on manyGeneral Motors double overhead camshaft engines on boththe intake and exhaust camshaft. The OCV directs pressurizedengine oil to either advance or retard chambers of thecamshaft actuator to change the camshaft position in relationto the crankshaft position. See Figure 24-61.




PADDLE

SPROCKET

ENGINE OILPRESSURE

PADDLECAVITY

SPROCKET

CAMSHAFT

OIL CONTROL VALVE (OCV)

ADVANCERETARD

FIGURE 24-60 A vane phaser isused to move the camshaft using changes of oilpressure from the oil control valve.

CONTROL VALVE

ELECTROMAGNET

RETURN SPRING

DRIVESPROCKET

DRIVECHAIN

VANE

VARIABLERANGE

FIGURE 24-61 A magnetically controlled vane phaser.

The following occurs when the pulse width is changed:

■ 0% pulse width—The oil is directed to the advancechamber of the exhaust camshaft actuator and the retardchamber of the intake camshaft actuator.

■ 100% pulse width—The oil is directed to the retardchamber of the exhaust camshaft actuator and the ad-vance chamber of the intake camshaft actuator.

The cam phasing is continuously variable with a rangefrom 40 degrees for the intake camshaft and 50 degrees for theexhaust camshaft. The ECM uses the following sensors to de-termine the best position of the camshaft for maximum powerand lowest possible exhaust emissions.

■ Engine speed (RPM)■ MAP sensor

■ Crankshaft position (CKP)■ Camshaft position (CMP)■ Barometric pressure (BARO)

Cam-in-Block Engine Cam Phaser

Overhead valve engines that use a cam-in-block design use a magnetically controlled cam phaser to vary thecamshaft in relation to the crankshaft. This type of phaseris not capable of changing the duration of valve opening orvalve lift.

Inside the camshaft actuator is a rotor with vanes that areattached to the camshaft. Oil pressure is supplied to the vanes,which causes the camshaft to rotate in relation to the crank-shaft. The camshaft actuator solenoid valve directs the flow ofoil to either the advance or retard side vanes of the actuator.See Figure 24-62.

The ECM sends a pulse-width-modulated (PWM) signalto the camshaft actuator magnet. The movement of the pintleis used to direct oil flow to the actuator. The higher the dutycycle, the greater the movement in the valve position andchange in camshaft timing.

NOTE: When oil pressure drops to zero when the engine stops, a spring-

loaded locking pin is used to keep the camshaft locked to prevent noise at

engine start.When the engine starts, oil pressure releases the locking pin.

VARIABLE VALVE

TIMING AND LIFT

Many engines use variable valve timing in an effort to improvehigh-speed performance without the disadvantages of a


466 CHAPTER 24


high-performance camshaft at idle and low speeds. There aretwo basic systems including:

■ Variable camshafts such as the system used byHonda/Acura called Variable Valve Timing and LiftElectronic Control or VTEC. This system uses twodifferent camshafts for low and high RPM. When theengine is operating at idle and speeds below about4000 RPM, the valves are opened by camshafts that areoptimized by maximum torque and fuel economy.When engine speed reaches a predetermined speed, de-pending on the exact make and model, the computerturns on a solenoid, which opens a spool valve. When thespool valve opens, engine oil pressure pushes againstpins that lock the three intake rocker arms together.With the rocker arms lashed, the valves must follow theprofile of the high RPM cam lobe in the center. Thisprocess of switching from the low-speed camshaft pro-file to the high-speed profile takes about 100 milliseconds(0.1 sec). See Figures 24-63 and 24-64.

■ Variable camshaft timing is used on many engines includ-ing General Motors 4-, 5-, and 6-cylinder engines, aswell as engines from BMW, Chrysler, and Nissan. On asystem that controls the intake camshaft only, thecamshaft timing is advanced at low engine speed, clos-ing the intake valves earlier to improve low RPM torque.At high engine speeds, the camshaft is retarded by usingengine oil pressure against a helical gear to rotate thecamshaft. When the camshaft is retarded, the intakevalve closing is delayed, improving cylinder filling athigher engine speeds. See Figure 24-65. Variable camtiming can be used to control exhaust cam timing only.Engines that use this system, such as the 4.2-liter GM in-line 6-cylinder engines, can eliminate the exhaust gas re-circulation (EGR) valve because the computer can closethe exhaust valve sooner than normal, trapping some ex-haust gases in the combustion chamber and thereforeeliminating the need for an EGR valve. Some engines usevariable camshaft timing on both intake and exhaustcylinder cams.

OIL FEED HOLES (4)

CAMSHAFTCAMSHAFT POSITION (CMP)ACTUATOR SOLENOID VALVE

SPOOLVALVE

SPOOL SPRING

FILTER SPRING

CHECKBALL

FIGURE 24-62 A camshaft position actuator used in a cam-in-block engine.

FIGURE 23-63 A plastic mock-up of a Honda VTEC system that usestwo different camshaft profiles; one for low-speed engine operation and theother for high speed.

INTAKECAMSHAFT

OUTERFOLLOWER

LOCKINGPINASSEMBLY

EXHAUSTCAMSHAFT

CENTERFOLLOWER

OUTERFOLLOWEROUTER

CAMLOBE

CENTERCAMLOBE

OUTERCAMLOBE

FIGURE 24-64 Engine oil pressure is used to switch cam lobes onthis VTEC system.




SOLENOID

OIL DRAINPORT

TIMING CHAINSPROCKET

PISTON

INTAKECAMSHAFT

SPRING

ENGINE OILPORT

HELICALGEAR

PLUNGER

FIGURE 24-65 A typical variable cam timing control valve. The solenoid is controlled by the engine computer and directs engine oil pressure to move a helical gear, which rotates the camshaft relative to the timing chain sprocket.

LIFTERS OR TAPPETS

Valve lifters or tappets follow the contour or shape of thecamshaft lobe. This arrangement changes the cam motion toa reciprocating motion in the valve train. Most older-stylelifters have a relatively flat surface that slides on the cam.

Most lifters, however, are designed with a roller to followthe cam contour. Roller lifters are used primarily in productionengines to reduce valve train friction (by up to 8%). This fric-tion reduction can increase fuel economy and help to offsetthe greater manufacturing cost. All roller lifters must use aretainer to prevent lifter rotation. The retainer ensures thatthe roller is kept in line with the cam. If the retainer broke, theroller lifter could turn, destroying both the lifter and thecamshaft. See Figures 24-66 and 24-67.

Valve train clearance is also called valve lash. Valve trainclearance must not be excessive, or it will cause noise or re-sult in premature failure. Two methods are commonly used tomake the necessary valve clearance adjustments. One in-volves a solid valve lifter with a mechanical adjustment, andthe other involves a lifter with an automatic hydraulic adjust-ment built into the lifter body called a hydraulic valve lifter.

SOLID LIFTERS

Overhead valve engines with mechanical lifters have an adjustment screw at the pushrod end of the rocker arm or an

FLAT TAPPET

ROLLERTAPPET

FIGURE 24-66 The camshaft lobe profile is totally different for anengine that uses a roller lifter compared to a flat-bottom lifter.

adjustment nut at the ball pivot. Adjustable pushrods are avail-able for some specific applications.

Valve trains using solid lifters must run with some clear-ance to ensure positive valve closure, regardless of the enginetemperature. This clearance is matched by a gradual rise in thecam contour called a ramp. (Hydraulic lifter camshafts do nothave this ramp.) The ramp will take up the clearance beforethe valve begins to open. The camshaft lobe also has a closingramp to ensure quiet operation.


468 CHAPTER 24


A lifter is solid in the sense that it transfers motion di-rectly from the cam to the pushrod or valve. Its physical con-struction is that of a lightweight cylinder, either hollow orwith a small-diameter center section and full-diameter ends.In some types that transfer oil through the pushrod, the ex-ternal appearance is the same as for hydraulic lifters. SeeFigure 24-68.

HYDRAULIC LIFTERS

A hydraulic lifter consists primarily of a hollow cylinder bodyenclosing a closely fit hollow plunger, a check valve, and apushrod cup. Lifters that feed oil up through the pushrodhave a metering disk or restrictor valve located under thepushrod cup. Engine oil under pressure is fed through an en-gine passage to the exterior lifter body. An undercut portionallows the oil under pressure to surround the lifter body. Oil

under pressure goes through holes in the undercut sectioninto the center of the plunger. From there, it goes downthrough the check valve to a clearance space between thebottom of the plunger and the interior bottom of the lifterbody. It fills this space with oil at engine pressure. Slight leak-age allowance is designed into the lifter so that the air canbleed out and the lifter can leak down if it should becomeoverfilled. The operating principle of a hydraulic lifter isshown in Figures 24-69 through 24-71.

PLUNGER

LOCKRING

PUSHRODCUP

CHECKBALL

CHECKVALVE

ROLLER

BODY

FIGURE 24-67 Many engines use hydraulic roller lifters to reducefrictional losses and improve fuel economy.

ALIGNINGYOKE

ROLLERLIFTER

ALIGNINGYOKE

YOKERETAINER

FIGURE 24-68 Roller lifters must be used with a part that keepsthe lifters from rotating as they move up and in the lifter bore.

LIFTERBODY RETAINING

RING

PUSHRODSEAT

OIL METERINGVALVEOIL ENTERS

LIFTER BODYHERE

FIGURE 24-69 A cutaway of a flat-bottom solid lifter. Because thistype of lifter contains a retaining ring and oil holes, it is sometimes confusedwith a hydraulic lifter that also contains additional parts.The holes in this lifterare designed to supply oil to the rocker arms through a hollow pushrod.




SNAP RING

PUSHROD SEAT

VALVE LIFTERBODY

OIL INLETS

OIL CHAMBER

PLUNGER

FEED HOLE

BALL RETAINERSPRING

PLUNGERSPRING

BALL RETAINER

FIGURE 24-70 Cutaway of a typical flat-bottom hydraulic valvelifter.

The pushrod fits into a cup in the top, open end of thelifter plunger. Holes in the pushrod cup, pushrod end, and hol-low pushrod allow oil to transfer from the lifter piston center,past a metering disk or restrictor valve, and up through thepushrod to the rocker arm. Oil leaving the rocker arm lubri-cates the rocker arm assembly.

As the cam starts to push the lifter against the valve train,the oil below the lifter plunger is squeezed and tries to returnto the lifter plunger center. A lifter check valve, either ball ordisk type, traps the oil below the lifter plunger. This hydrauli-cally locks the operating length of the lifter. The hydraulic lifterthen opens the engine valve as would a solid lifter. When thelifter returns to the base circle of the cam, engine oil pressureagain works to replace any oil that may have leaked out of thelifter.

The hydraulic lifter’s job is to take up all clearance in thevalve train. Occasionally, engines are run at excessivespeeds. This tends to throw the valve open, causing valvefloat. During valve float, clearance exists in the valve train.The hydraulic lifter will take up this clearance as it is de-signed to do. When this occurs, it will keep the valve fromclosing on the seat. This is called pump-up. Pump-up willnot occur when the engine is operated in the speed range forwhich it is designed.

FrequentlyAsked Question

HOW DOES CYLINDERDEACTIVATION WORK?

Some engines are designed to be operated on four ofeight or three of six cylinders during low load conditionsto improve fuel economy.

The power train computer monitors engine speed,coolant temperature, throttle position, and load and de-termines when to deactivate cylinders. The key to thisprocess is the use of two-stage hydraulic valve lifters. Innormal operation, the inner and outer lifter sleeves areheld together by a pin and operate as an assembly. Whenthe computer determines that the cylinder can be deacti-vated, oil pressure is delivered to a passage, which de-presses the pin and allows the outer portion of the lifterto follow the contour of the cam while the inner portionremains stationary, keeping the valve closed. The elec-tronic operation is achieved through the use of lifter oilmanifold containing solenoids to control the oil flow,which is used to activate or deactivate the cylinders. Gen-eral Motors used to call this system Displacement on De-mand (DOD), but now calls it Active Fuel Management.Chrysler calls this system Multiple Displacement System(MDS). See Figures 24-72 and 24-73 on pages 470–471.

LIFTER PRELOAD

Lifter preload is actually the distance between the pushrodseat inside the lifter and the snap ring of the lifter when thelifter is resting on the base circle (or heel) of the cam and thevalve is closed. This distance should be about 0.020 to 0.045inch. On engines with adjustable rocker arms, this distance orpreload is determined by turning the rocker arm adjusting nutone-quarter to one full turn after zero lash (clearance) is deter-mined. See Figure 24-74 on page 471.

Tightening this adjusting nut further can cause thepushrod to bottom out in the lifter. If the engine is rotated withthe pushrod bottomed out, bent valves, as well as bent or dam-aged pushrods, rocker arms, or rocker arm studs can result.

Engines that have been rebuilt or repaired and that do notuse adjustable rocker arms are particularly at risk for damage.If any of the following operations have been performed, lifterpreload must be determined:

■ Regrinding the camshaft (reduces base circle dimensions)■ Milling or resurfacing cylinder heads


470 CHAPTER 24


■ Milling or resurfacing block deck■ Grinding valves and/or facing valve stems■ Changing to a head gasket thinner or thicker than the

original

Most lifters can accept a total variation in the entire valvetrain of about 0.080 to 0.180 inch. To determine lifterpreload, rotate the engine until the valve being tested isresting on the base circle of the cam. For example, with thevalve cover off and rotating the engine in the normal operat-ing direction, watch the exhaust valve start to open. Thismeans that the intake valve for that cylinder is resting on thebase circle (heel) of the cam. Apply pressure down on thelifter. Wait several minutes for the lifter to bleed down. Mea-sure the distance between rocker arm and valve stem. If theproper clearance is not obtained (generally between 0.020and 0.045 inch), the following may need to be done to get theproper clearance:

1. Install longer or shorter pushrods. Manufacturers pro-duce pushrods in various lengths. Some are available inlengths up to 0.100 inch longer or shorter than stock.

2. Install adjustable pushrods or rocker arms, if possible.3. Shim or grind rocker stands or shafts.

NOTE: Shim rocker arm supports to three-fifths of the measurement

removed from the cylinder head (1.5:1 rocker ratio). For example, if

0.030 inch is removed from a cylinder head,shim the rocker arm to 0.030 inch

times 3/5, or 0.018 inch.

PLUNGER

LOCKRING

PUSHRODCUP

CHECKBALL

CHECKVALVE

ROLLER

BODY

FIGURE 24-71 An exploded view of a hydraulic roller lifter.

UNAPPLIED PRESSURE

APPLIED

PRESSURE

SPRING PUSHES THELOCKING PIN OUTWARD

ENGINE OIL PRESSURE PUSHES THE LOCKING PIN INWARD

LIFTER ENABLED

LIFTER DISABLED

FIGURE 24-72 Oil pressure applied to the locking pin causes the inside of the lifter to freely move inside the outer shell of the lifter, thereby keeping the valve closed.




A properly adjusted valve train should position the lifter in thecenter of its travel dimension.

The procedure for a valve train with adjustable rockerarms is as follows:

1. Rotate the engine clockwise as viewed from the nonprin-cipal or belt end (normal direction of rotation) until theexhaust lifter starts to move up.

2. Adjust the intake valve to zero lash (no preload) and thenone-half turn more.

3. Rotate the engine until the intake valve is almost com-pletely closed. Adjust the exhaust valve to zero lash andthen one-half turn more.

4. Continue with this procedure for each cylinder until allthe valves are correctly adjusted.

If the valve train uses nonadjustable rocker arms, thelifter preload must still be determined. The lifter preload mustbe measured if any or all of the following procedures havebeen performed on the engine:

■ Head(s) milled■ Block decked■ Valves ground■ Any other machining operation that could change the

valve train measurement

Shorter (or longer) replacement pushrods may be re-quired to produce the correct lifter preload. Some engine man-ufacturers recommend using thin metal shims under therocker arm supports if needed.

CAUTION: If shims are used under the rocker arm supports, be sure that

the shim has the required oil holes.

DETERMINING PROPER

LIFTER TRAVEL

To determine if shimming or use of replacement pushrods ofdifferent lengths is required, use the following procedure:

1. With the valve cover removed, rotate the engine untilthe valve lifter being tested is resting on the base circleof the camshaft.

2. Depress the pushrod into the lifter with steady pressure.This should cause the lifter to bleed down until thepushrod bottoms out in the lifter bore.

3. Measure clearance (lash) between the rocker arm tip andthe stem of the valve. This measurement varies accord-ing to manufacturer and engine design, but it usuallyranges from 0.020 to 0.080 inch. See Figure 24-75. Always consult exact manufacturer’s specifications be-fore taking any corrective measures.

HIGH-CAPACITYGEROTOR PUMP

LIFTER OIL MANIFOLDASSEMBLY

TWO-STAGELIFTER

FIGURE 24-73 Active fuel management includes many differentcomponents and changes to the oiling system,which makes routine oil changeseven more important on engines equipped with this system.

FIGURE 24-74 Operating principle of hydraulic lifters.

DETERMINING LIFTER

PRELOAD

The process of adjusting valves that use hydraulic valve liftersinvolves making certain that the lifter has the specified preload.


472 CHAPTER 24


PUSHROD

CAMSHAFT

HYDRAULICLIFTER

MEASURE HERE(BETWEEN ROCKERARM AND VALVE STEM)

PUSH DOWN

ROCKER ARM

FIGURE 24-75 Procedure for determining proper lifter travel.

If the measurement is not within acceptable range, selectthe proper length pushrods to achieve the proper lifter traveldimension and preload.

NOTE: Some engines use several different pushrod lengths depending on

the exact build date! Block casting numbers may be the same, but the en-

gines may require different internal parts. Check with the manufacturer’s

specifications in the factory service manual for proper interchangeable parts.

VALVE NOISE DIAGNOSIS

Valve lifters are often noisy, especially at engine start-up.When the engine is off, some valves are open. The valve springpressure forces the inner plunger to leak down (oil is forcedout of the lifter). Therefore, many vehicle manufacturers con-sider valve ticking at one-half engine speed after start-up to benormal, especially if the engine is quiet after 10 to 30 seconds.Be sure that the engine is equipped with the correct oil filter,and that the filter has an internal check valve. If in doubt, usean original-equipment oil filter. If all of the valves are noisy,check the oil level. If low, the oil may have been aerated (airmixed with the oil), which would prevent proper operation ofthe hydraulic lifter. Low oil pressure can also cause all valvesto be noisy. The oil level being too high can also cause noisyvalve lifters. The connecting rods create foam as they rotatethrough the oil. This foam can travel through the oiling systemsto the lifters. The foam in the lifters prevents normal operationand allows the valves to make noise.

If the valves are abnormally noisy, remove the valve armcover and use a stethoscope to determine which valves orvalve train parts may be causing the noise. Check for all of thefollowing items:

■ Worn camshaft lobe■ Dirty, stuck, or worn lifters■ Worn rocker arm (if the vehicle is so equipped)■ Worn rocker arm shaft (See Figure 24-76.)■ Worn or bent pushrods (if the vehicle is so equipped)■ Broken or weak valve springs■ Sticking or warped valves

MECHANICAL LIFTER

SERVICE

Mechanical lifters, like hydraulic lifters, should be replaced ifthe camshaft is replaced. If the lifters are to be reused, theymust be kept in order and reinstalled in the exact positions inwhich they were originally used in the engine. All liftersshould be cleaned and carefully inspected. If the base of thelifter is dished (concave), the lifter should be replaced. Aswith any lifter, new or used, the bore clearance should bechecked.

NOTE: Regrinding of valve lifter bases is generally not recommended

because the hardened areas of the lifter can be ground through.

SPRING

ROCKERARM

ROCKER ARMSHAFTS

FIGURE 24-76 Shaft-mounted rocker arms are held in position byan assortment of springs,spacers,and washers which should be removed so thatthe entire shaft can be inspected for wear.




HYDRAULIC LIFTER

SERVICE

Hydraulic lifter service begins with a thorough visual inspec-tion. Compare the lifter wear with the corresponding lobe onthe camshaft. All lifters should be replaced during a major en-gine overhaul or a camshaft replacement.

Vehicle manufacturers usually recommend that, becauseof their high cost, hydraulic roller lifters be checked for wear,disassembled, and cleaned rather than being replaced. Anyother hydraulic lifter that is to be reused should also be disas-sembled and cleaned using the following steps:

Step 1 Select a clean work area and tray for the disassem-bled parts. See Figure 24-77.

Step 2 Disassemble the lifters and keep all parts in order.Step 3 Clean all parts and reassemble. Always use a lint-

less cloth because lint can affect lifter operation.Step 4 Test leak-down rate using a leak-down tester and

special-viscosity fluid. See Figure 24-78.a. Measure the time required for the fluid to pass

between the inner and outer body of the lifter.b. The time it takes for the lifter to collapse under a

given weight should be longer than 10 secondsand less than 90 seconds.

c. Check service information for the exact leak-down time for your vehicle. The average time forleak-down is 20 to 40 seconds.

HYDRAULIC VALVE LIFTER

INSTALLATION

Most vehicle manufacturers recommend installing lifterswithout filling or pumping the lifter full of oil. If the lifter is filledwith oil during engine start-up, the lifter may not be able to

bleed down quickly enough and the valves may be kept open.Not only will the engine not operate correctly with the valvesheld open, but the piston could hit the open valves, causing se-rious engine damage. Most manufacturers usually specify thatthe lifter be lubricated. Roller hydraulic lifters can be lubricatedwith engine oil, whereas flat lifters require that engine assem-bly lube or extreme pressure (EP) grease be applied to the base.

BLEEDING HYDRAULIC

LIFTERS

Air trapped inside a hydraulic valve lifter can be easily bled bysimply operating the engine at a fast idle (2500 RPM). Normaloil flow through the lifters will allow all of the air inside thelifter to be bled out.

NOTE: Some engines, such as many Nissan overhead camshaft engines,

must have the air removed from the lifter before installation. This is accom-

plished by submerging the lifter in a container of engine oil and using a

straightened paper clip to depress the oil passage check ball.

Consult a service manual if in doubt about the bleedingprocedure for the vehicle being serviced. See Figure 24-79 foran example of the special tool needed to bleed out the air onhydraulic lash adjusters (HLA) used on a Chrysler 3.2/3.5 L,V-6 overhead camshaft.

VALVE TRAIN LUBRICATION

The lifters in an overhead valve (OHV) engine are lubricatedthrough oil passages drilled through the block. The engine oilthen flows through the lifter, and up through the hollowpushrod where the oil flows over the rocker arms to lubricateand cool the valve and valve spring.

FIGURE 24-77 Cleaning a disassembled hydraulic lifter (tappet) inclean petroleum solvent. After cleaning and reassembly, all hydraulic liftersshould be checked for proper leak-down rate using a special fluid and tester.

POINTER

WEIGHTED ARM

CUP

RAMPUSHROD

FIGURE 24-78 Typical hydraulic lifter leak-down tester.


474 CHAPTER 24


NEEDLE TOOL

HYDRAULICLASH ADJUSTER

FIGURE 24-79 A special tool with a small needlelike probe is usedto bleed air from the hydraulic lash adjuster (HLA).

ROCKER ARMSHAFT

OIL SUPPLYFROM HOLLOWPUSHROD

FIGURE 24-80 Hollow pushrods supply oil to pushrod ends, rockerarms, and shafts. Oil then drips from the rocker arm onto the valve springs andreturns to the oil pan through oil drain holes in the cylinder head.

An old or rebuilt engine that uses flat-bottom liftersmust use one of two lubricants:

1. Oils that contain at least 0.15% or 1,500 PPMof zinc in the form of ZDDP. Oils that containthis much zinc are designed for off-road use onlyand in a vehicle that does not have a catalyticconverter, such as racing oils. If the vehicle isequipped with a catalytic converter, replace thecamshaft and lifters to roller-type so that neweroils with lower levels of zinc can be used.

2. Use a newer oil and an additive such as:a. GM engine oil supplement (EOS)b. Comp Cams® camshaft break-in oil additive,

Part #159c. Crane Cams® Moly Paste, Part #99002-1d. Crane Cams® Super Lube oil additive,

Part #99003-1e. Lumati Assembly lube, Part #99010f. Mell-Lube camshaft tube oil additive,

Part #M-10012

TechTipTWO CHOICES

IF USING FLAT-

BOTTOM LIFTERS

NOTE: The Chrysler 5.7-liter Hemi engine is opposite because the oil is

first sent to the rocker arm through passages in the block and head and then

down through the hollow pushrod to the lifters.

See Figure 24-80.Other engine designs supply engine oil to the rocker

arms and valves through passages in the block and head. SeeFigure 24-81.

OILGALLERY

TO MAIN THEBEARINGSROCKER SHAFT

OIL PASSAGE

FIGURE 24-81 Oil galleries (passages) often supply oil to the rockershafts.




FIGURE 24-82 A flat-head Ford V-8 engine. Note that the exhaustmanifold has only three exhaust ports.The center two cylinders shared one port.

FIGURE 24-83 A cutaway of an early Chrysler Hemi showing thevalves and hemispherical combustion chamber.

Summary

1. The camshaft rotates at one-half the crankshaft speed.2. The pushrods should be rotating while the engine is run-

ning if the camshaft and lifters are okay.3. On overhead valve engines, the camshaft is usually

placed in the block above the crankshaft. The lobes of thecamshaft are usually lubricated by splash lubrication.

4. Silent chains are quieter than roller chains but tend tostretch with use.

5. The lift of a cam is usually expressed in decimal inchesand represents the distance that the valve is lifted off thevalve seat.

6. In many engines, camshaft lift is transferred to the tip ofthe valve stem to open the valve by the use of a rockerarm or follower.

7. Pushrods transfer camshaft motion upward from thecamshaft to the rocker arm.


WHAT IS A FLAT HEAD ENGINE?

Most early engines had the valves placed in the blockoperated by the cam-in-block camshaft. This type of en-gine design is called a flat head because the cylinderhead is flat and does not contain any coolant passagesor ports. In 1932, Ford introduced the first low-cost V-8 engine using the flat head or side valve design. Thisengine was used in early hot rods and even into the1960s after overhead valve (OHV) design engines werebecoming more popular and powerful. Old-timers usedto have a common expression about their Ford flat-head V-8s that said.

“Flat heads forever” and “Real engines do not havevalve covers.”

See Figure 24-82.


WHEN WAS THE FIRSTCHRYSLER HEMI?

The original Chrysler Hemi engine that used a combus-tion chamber with a hemispherical shape was offered asan option in 1951. The original engine design was over-square with a bore (3.81 in.) longer than the stroke(3.63 in.) for a total displacement of 331 cu. in. SeeFigure 24-83.



476 CHAPTER 24

8. Camshaft duration is the number of degrees of crankshaftrotation for which the valve is lifted off the seat.

9. Valve overlap is the number of crankshaft degrees forwhich both valves are open.

10. Camshafts should be installed according to the manufac-turer’s recommended procedures. Flat lifter camshafts shouldbe thoroughly lubricated with extreme pressure lubricant.

11. If a new camshaft is installed, new lifters should also beinstalled.

Review Questions

1. Explain why the lift and duration and lobe-center dimension ofthe camshaft determine the power characteristics of the engine.

2. Explain lobe centerline.

3. Describe the operation of a hydraulic lifter.

4. Describe how to adjust hydraulic lifters.

Chapter Quiz

1. The camshaft makes _____ for every revolution of the crankshaft.

a. One-quarter revolution

b. One-half revolution

c. One revolution

d. Two revolutions

2. Flat-bottom valve lifters rotate during operation because of the _____ of the camshaft.

a. Taper of the lobe

b. Thrust plate

c. Chain tensioner

d. Bearings

3. If lift and duration remain constant and the lobe center angledecreases _____.

a. The valve overlap decreases

b. The effective lift increases

c. The effective duration increases

d. The valve overlap increases

4. Which timing chain type is also called a “silent chain”?

a. Roller

b. Morse

c. Flat link

d. Both b and c

5. Two technicians are discussing variable valve timing. Technician Asays that changing the exhaust valve timing helps reduce exhaustemissions. Technician B says that changing the intake valve timinghelps increase low-speed torque. Which technician is correct?

a. Technician A only

b. Technician B only

c. Both Technicians A and B

d. Neither Technician A nor B

6. Many technicians always use new pushrods because _____.

a. It is less expensive to buy than clean

b. All of the dirt cannot be cleaned out from the hollow center

c. Pushrods wear at both ends

d. Pushrods shrink in length if removed from an engine

7. A DOHC V-6 has how many camshafts?

a. 4

b. 3

c. 2

d. 1

8. The intake valve opens at 39° BTDC and closes at 71° ABDC.The exhaust valve opens at 78° BBDC and closes at 47° ATDC.Which answer is correct?

a. Intake valve duration is 110°

b. Exhaust valve duration is 125°

c. Overlap is 86°

d. Both a and b

9. Hydraulic valve lifters can make a ticking noise when theengine is running if _____.

a. The valve lash is too close

b. The valve lash is too loose

c. The lobe centerline is over 110°

d. Both a and c

10. Hydraulic lifters or hydraulic lash adjusters (HLA) may notbleed down properly and cause an engine miss if _____.

a. The engine oil is one quart low

b. The wrong API-rated engine oil is used

c. The wrong SAE-rated engine oil is used

d. Both a and b


m24 hald6891 06 se ch24 - the eye · nose. sometimes a valve “tick, tick, tick” noise is heard...

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