chapter 12

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Chapter 12

Upper End Theory and Service


Cylinder Heads

• Made of cast iron or aluminum• Contains valves, valve seats, valve guides,

springs, rocker arm supports, and the upper portion of the combustion chamber

• Contains coolant and oil return passages• Head design is one of the most influential

factors in overall engine performance


Cylinder Heads (Cont.)


Cylinder Heads (Cont.)

• Intake and exhaust ports are cast into the head• Most heads are aluminum to reduce weight• Heads can have crossflow or Siamese port


Crossflow and Siamese Heads


• Wedge Chamber– The spark plug is located at the widest part of

the wedge• Hemispherical Chamber– The combustion chamber is shaped like a half

circle– The spark plug is located in the center of the

combustion chamber

Combustion Chamber Types


Combustion Chamber Types (Cont.)

• Pentroof Combustion Chamber– Used in many modern engines– Modified hemispherical chamber design– Found mostly on engines with four valves per

cylinder


Wedge Combustion Chamber


HemisphericalCombustion Chamber


Pentroof Combustion Chamber


Intake and Exhaust Valves

• Commonly called poppet valves• Allow air to flow into and exhaust out of the

combustion chamber• Seal the upper portion of combustion

chamber• Intake valves are larger than exhaust valves


Valve Construction

• Most valves are hardened steel, steel alloys, or stainless steel

• Intake and exhaust valves are usually made of different materials

• Valves can be one or two pieces• Some exhaust valves are sodium filled• Some high performance engines use

titanium valves


Valve Terminology


• May be cast into or inserted into head• Seals the valve and transfers heat

Valve Seats


Valve Guides

• Support the valve and allow only up and down motion

• Close clearance to valve stem• Keeps valve aligned with valve seat• Can be cast into or insert into the head


Valve Guides (Cont.)


Valve SpringRetainers and Oil Seals


Rocker Arms and Pushrods

• Change the direction of the cam’s lifting force

• Pushrods link the cam to the rocker arm


Cam Followers

• Used on some OHC engines

• Can be used to hydraulically adjust valve lash

• Increases contact between cam and valve


Camshaft Bearings

• Most OHC cylinder heads are machined for the camshaft to ride directly on the head


Knowledge Check

• Technician A says the part of the valve that moves through the guide is called the valve face. Technician B says the part of the valve that moves through the guide is called the valve stem. Who is correct?


Multivalve Engines

• Used to improve airflow in and out of combustion chamber

• Larger valves require increased spring tension• Three, four, and five valves per cylinder used– Four is most common


Variable Valve Timing

• VVT can be intake only, intake and exhaust, or vary lift and timing of all valves

• Staged systems allow two valve timing and lift settings – Honda’s VTEC

• Continuously variable systems change cam phasing


VVT Phaser


Fiat MultiAir

• Provides variable lift and duration of intake valves

• PCM controlled hydraulic solenoid controls valve opening


BMW Valvetronic System

• Varies intake valve lift• Combined with

VANOS, provides variable lift and valve timing


Cylinder Deactivation

• Keeps valves closed on specific cylinders• Decreases the working displacement• OHC engines may lock and unlock the rocker

arms• OHV engines deactivate lifters – Chrysler and

GM displacement on demand


Cylinder Head Disassembly

• On some OHC engines, the rocker arms must be removed first

• Remove the camshaft• Keep bearing caps and follow assemblies in

order


Knowledge Check

• What are two advantages gained by using VVT systems in modern engines?


Cylinder Head Disassembly (Cont.)

• Measure spring installed height before disassembly


Cylinder Head Disassembly (Cont.)

• Use a socket and mallet to loosen the valve keepers

• Attach and adjust the valve spring compressor• Compress the spring and remove the keepers• Remove retainer, spring(s), and seal


Compressing Valve Springs


Cylinder Head Inspection

• Replace a head showing severe damage to valve sealing areas

• Check for dents, scratches, cracks, and corrosion damage

• Check intake and exhaust manifold mounting surface flatness


Measure Head Warpage


Cylinder Head Inspection (Cont.)

• Inspect aluminum heads for porosity leaks• Check head thickness and compare to

specifications• Check OHC style heads for proper cam bore

alignment


Valve Inspection

• Check each valve face for signs of burning

• Discard any valves that are burned, worn, bent, or has the plating flaking off or chipped

• Look for oil buildup on the back of the valve


Inspect the Stem for Wear


Valves Seats

• Inspect for damage, burning, cracks, and deterioration

• Check for loose seats• Inspect for sunken valve seats• Seats should be replaced if the valve was

broken or bent


Checking for Loose Valve Seats


Retainers and Keepers

• Worn retainers will allow the spring to move away from the valve centerline

• Uneven wear on the stem tip indicates rotators not working properly


Valve Springs

• Check for cracks, breaks, damage, and rust• Perform tests to determine valve spring

condition– Freestanding height test– Squareness test– Open/close pressure test


(A) Freestanding height

(B) Closed valve spring height

(C) Open valve spring height


Inspection of the Valve Train• Timing Belts– Normally replaced when the engine or head is rebuilt– Ensure there are no fluid leaks that can damage the

belt

• Timing Chains– Measure chain for stretch

• Idler Pulleys– Check for smooth operation and lubricant leakage

• Tensioners– Check for smooth operation and lubricant leakage


• Gears and Sprockets– Check for cracks and tooth wear

• Cam Phasers– Perform air testing of advance and retard

action• Cam Followers and Lash Adjusters– Check for movement of shim

Inspection of the Valve Train (Cont.)


• Rocker Arms– Inspect contact points and oil feed hole

• Pushrods – Check oil passage and for straightness

• Camshaft and Bearings– Inspect lobe and bearing journals– Check cam straightness

Inspection of the Valve Train (Cont.)


Knowledge Check

• What are three conditions valve springs are checked for?


• Cracks are common between spark plug bore and valve seat

• Cracks between seats are also common

Servicing Cylinder Heads


• Resurface– Makes the sealing surface flat so the gasket

seals properly– Raises the compression ratio– Squares the deck to the main bores

Resurfacing a Cylinder Head


Stock Removal Guidelines

• Resurfacing reduces combustion chamber size – increasing compression ratio

• Reduced head thickness changes valve timing• May reduce valve-to-piston clearance• Intake manifolds on V-type engines may not

align


Valve Face

• Valve face angles and dimensions must be kept within specifications


Valve Stem

• Grinding the face sets the valve lower into the port – extending the stem height


Valve Stem (Cont.)

• If stem height is excessive, the valve stem tip may be ground

• Chamfer valve tip after grinding• Valve and/or seat may need to be replaced to

correct incorrect stem height


Valve Guide Clearance

• Measure the guide diameter and stem diameter


• Inadequate Lubrication– Is caused by oil starvation in the upper valve train

• Valve Geometry Problems– Lead to increased guide clearance, poor valve

seating, and premature wear• Wrong Stem to Guide Clearance– Can cause increased oil consumption

Valve Guide Problems


Valve Guide Reconditioning


Valve Guide Height

• Important to avoid interference with valve spring


Reconditioning Valve Seats

• Seat must be correct width and in correct location

• Important for heat transfer• Seating pressure reduced on wide seats• Seats may be ground or replaced


Measuring Seat Width


Valve Seat Grinding

• Seats are ground at an angle to the valve face

• Many seats are cut with three angles


Checking Valve Fit

• Coat valve face with prussian blue• Snap valve open and closed against seat• Inspect the pattern of the dye


Valve Lapping

• Uses an abrasive compound to remove small imperfections in valve face/seat contact area


Valve Stem Seals

• Control the amount of oil between the stem and guide


Knowledge Check

• A valve seat that is too narrow may cause insufficient transfer from the valve to the head.


Assembling the Cylinder Head

• Installed stem height and installed spring height must be checked

• Stem height affected by valve and seat• Spring height may be adjusted using shims


Assembling OHC Heads

• Camshaft full round insert bearings are pressed into the head

• Do not use an OHV cam bearing tool• Some OHC engines do not use insert

bearings• Coat all rockers, adjusters, pushrods, lifters,

and other parts in oil for assembly• Adjust valve lash and install head

chapter 12

Documents

cengage learning

accessible website

wedge combustion chamber

piecessome exhaust valves

combustion chamberseal

combustion chamber types

aluminumcontains valves

titanium valves