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The Thumper 600 kit looks stock fromthe outside, but when you crack thethumb throttle, it will warp your world!

This KTM has enough power to light upthe Skat Trak Haulers and sling it side-

ways at a blip of the throttle.

ing assembly is balanced to the newparts to achieve a quick-revvingsmooth engine, producing moretorque than you can wrap your headaround. To allow the engine to breathe,spec head porting, 1mm-plus over-sized valves and a 43mm Keihin FCRcarburetor are hooked up to a FuelCustoms intake and FMF Factory 4.1exhaust.

OTHER GOODIESThe KTM we tested was running

black ITP wheels with ITP Sand Starfront tires and Skat Trak Hauler rearpaddles. The owner of the KTM alsocustom built a titanium-lined oil cool-er using a billet adapter that replacesthe second oil filter on the KTM RFSengine and routes hot oil past theengine to a cooler position behind thesteering stem. This ups the reliabilityeven further and plays a major partin keeping that Thumper motor fresh.The stock KTM XC arms werereplaced with KTM’s OEM widerarms, and the shocks were revalvedby Rocket Ron for a plusher ride.Fasst Company Flexx bars wereinstalled, as well as a Precision steer-ing stabilizer and Pro Armorbumper/nerf package. Overall, it’s agreat-looking quad that looks rela-tively stock—but doesn’t sound like it.

TAMING THE BEASTFrom the first time we fired up the

600cc earthmover, we were in love.The stock battery and starter spin themotor to life just as effortlessly asstock, and it starts and purrs at idlewith just a slighty more aggressivecam lope than stock. When you blipthe throttle, the “bop bop bop” of thebig single engine quickly snaps to lifelike a clap of thunder in an intensestorm, cracking off revs quicker thanyou could imagine for such a largepiston. Clutch engagement is just likestock, so pulling away from a stop isabout as simple as cracking a rev anddropping the throttle, letting thetorque do the work for you. The 600will lope around near idle all day, andfrom there on out, things start to blur.Stuffing the throttle into the stops fromany low speed is a fruitless effort, as iteither ends in too much wheelspin ora 90-degree wheelstand. It could real-

ly use an extended swingarm to keepthe front end down, but doing sowould slow its handing slightly. Rollinto it and keep your weight forwardin first and second, and it moves outquick enough to make you think twiceabout shifting into third under power.However, the power is so linear andsmooth that full-throttle upshifts fromthird to fifth don’t produce anythingmore than a power-induced wheel-stand and a ton of roost. It hits lightspeed in a hurry and will easily standup without any weight shift in anygear. It’s like holding all of the world’spower in your hands—use it wisely orit will bite you.We took the KTM to the dirt and the

dunes, spending the majority of ourtesting time in the soft sand at PismoBeach, California. Jetting at sea levelwas spot-on, and the KTM ran with-out any hiccups. Riding through thedunes is still just as fun as it is on the

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Thumper Racing, $4729www.thumperracing.com• 600XC built by Travis Smith at

Thumper Racing.• Stroked, lightened and balancedcrankshaft: $1650

• Thumper Racing stroker 99mm big-bore kit, 12.5 to 1. Includes cylindersleeve and replate, CP 12.5:1 pis-ton with ceramic coating, gasketsand jetting: $775

• Head porting by Thumper Racing:$350

• +1mm stainless steal kibble whitevalves: $164; valve springs with tita-nium keepers: $199; Hot Cam: $189

• 43mm carburetor jetted for 600ccmotor: $350

• Engine gasket set: $180• Carillo rod: $275• Stage 5 cam• High-flow water pump: $145• Engine assembly labor: $640KTM OEM Parts• Extended KTM “SX” A-arms• KTM billet ¼-turn gas cap• KTM billet front sprocket cover• KTM billet oil filter cap• KTM billet brake reservoir cover• KTM billet hydraulic clutch reser-

voir coverFMF Racingwww.fmfracing.com, (310) 631-4363

• Megabomb full-titanium exhaustsystem 4.1 exhaustCP Pistonswww.cppistons.com, (949) 567-9000• High-compression big-bore piston

supplied to ThumperCometic Gasketswww.cometic.com, (800) 752-9850• Big-bore head gasket supplied toThumper

Millennium Techwww.mt-llc.com, (920) 893-5595• Over-bore and replating supplied

to ThumperHot Camswww.hotcamsinc.com, (515) 402-8005• Stage 1 Hot Cam supplied to

ThumperDunlop Tirewww.dunlopmotorcycle.com,(800) 845-8378• TiresITPwww.itptires.com, (909) 390-1905• WheelsCV4www.cv4.net, (800) 874-1223• Formed silicone coolant hose,orangePro Armorwww.proarmor.com, (888) 312-7667• Front bumper• Pro Peg sport nerf bars

Rocket Ron Racingwww.rocketronracing.com,(317) 319-4381• Rebuilt/revalved and upgraded

triple-rate Ohlins shocksFour Werx Carbonwww.fwcarbon.com, (262) 501-9696• Carbon fiber carburetor heat

shield• Carbon fiber rear shock

heat shield• Carbon fiber exhaust shield• ASV pro clutch leverPrecision Racing Productswww.precision-rp.com,(209) 365-1850• Steering stabilizerSRC SchulerRacing Componentswww.schulermotorsports.com,(619) 813-4873• Oil cooler mounting bracket bySRC

• Titanium plumbing for the oil cool-er by SRC

• Jagg 10 row oil cooler• Billet clamps for oil cooler• Minot billet water pump• Polaris bob oil filter adapter• Extended brake pedal• Crankcase pressure-evacuationvalve

• AMSOIL motor oil

KTM 525XC WITH THUMPER 600 KIT

The companies who helped build thisbeast are cleanly displayed in a beauti-ful DeCal Works graphics kit, completewith engine-size labels!

but you can ride it to and from CompHill, and that we like.

WORDS FROM THEENGINE BUILDER

Travis Smith, owner of ThumperRacing:“We set out to build a motor that

would run on pump gas and makeawesome power. We first looked atthe limits of the stock KTM RFS motor.For this build we didn’t want to getinto boring cases, so we increaseddisplacement via stroke and bore andcompression. We took our tried-and-true 99mm (4mm-plus), 12:1 compres-

stock KTM XC, albeit with much morethump behind your thumb. Crack thethrottle and it practically skips acrossthe surface of the sand at speed, butit’s tame, controllable and oh-so-smooth. The torque curve is nearelectric in its tractability, with a linearbuild that stretches your arms butwon’t tear them from your sockets.Basically, it feels like the stock 525power curve but amplified by nearlydouble. And with power numberscoming in a little shy of 68 wheel

horsepower, it’s no joke. We eked a lit-tle over 70 mph out of the KTM on thetop end, and the power could pullgearing tall enough to get it near 100mph, we’re guessing. Out at the sanddrags, there wasn’t a thing we cameup against that would give it a run forits money—other big-bore 450s, mod-ded dirt bikes, built Banshees. Thetorque pulled it far ahead of the com-petition on the uphill drag strip.Obviously it’s not going to take out afull-drag, 140-horsepower Banshee,

It’s much faster than stock butjust as light, so tossing the 600around in the air is no problem.

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Rocket Ron revalved the stock Ohlinsshocks, and KTM’s extended A-arm kitwidens the front for more stability anda stock look.

sion, big-bore kit and then added a6mm increase in stroke, giving themotor a 600cc displacement. Becauseof the stroke increase, the 12.5:1 com-pression piston was now at 12.9:1,which is the limit of compression onpremium pump gas. The piston wasceramic-coated to keep the motor run-ning cooler by insulating the pistonfrom the combustion chamber.“Normally any increase in stroke

slows the motor and affects handling.In order to offset this and to keep themotor vibration free, we reshaped andlightened the crank by 1400 grams andbalanced the crank to the larger pis-ton. Lightening and balancing keepthemotor vibrations to aminimum andrevving out, as well as stock plus theincrease in gyroscopic forces to a min-imum. We also chose to use the stock-length connection rod and offset thepiston wrist pin in the piston instead ofshortening the rod as most strokercranks do. By keeping the stock rodlength, we keep the rod-to-stroke ratiosbetter, which ultimately makes bettertorque, decreases piston accelerationspeed and piston side load, adding tomotor reliability and longevity.“In order to keep airflow into the big

motor, the head received our Stage 3porting, oversized high-flow valves,Stage 4 racing cam and high-lift valvesprings. To get the full benefit of thecam, it must be timed to the crank, asfactory indexing can vary 1 to 3degrees. The carburetor was over-bored from the stock 39mm bore to43mm bore and rejetted to match the600cc displacement.” �

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