There is nothing to fear

By Allen Knowles  and staff of Dirt Wheels


When performance quads were two-strokes without power valves, rebuilding an engine was relatively cheap, even with labor included. Modern two-strokes with power valves are definitely more expensive to work on, but those are not available for quads unless you are willing to build your own. With modern quads, it doesn’t matter whether you are working on a sport quad or a 4×4, you will be working on a four-stroke.

Costs soar with the increased complexity of four-strokes. On the plus side, 4×4 engines have an extremely robust lifespan, and relatively low-revving engines like the Raptor 700 last a good, long time. High-performance, high-rpm engines like modern 450cc sport quads need more attention and preventative maintenance. It is critical that you replace parts before they break.

When an engine breaks apart at 11,000 rpm, there will be collateral damage. These modern engines use plated cylinders and short-skirt, slipper-style pistons. If you ride hard, routine piston changes and a schedule of crankshaft changes will usually keep the carnage in check.

WHAT IS INCLUDED: Most of the kits have what we call the automatics, the things you would always change if you have the cases split. That would include main bearings, seals, cam chain, and a rod kit or crank. There are other parts that need to be inspected to determine whether they need to be replaced. The inspection list should include the oil pump, transmission gears, shift forks, clutch, clutch basket, cam chain sliders, counter-balancer, and all of the engine and transmission bearings.





Many companies are offering kits that include every part you need to perform a complete rebuild. Usually, these kits do not include new valve-train parts. Having every single part, seal and gasket really do make the process easier. If you’ve been thinking about tackling a rebuild, we thought we would offer you some tips to help you get through a rebuild and skip some beginner mistakes.

Steps that you must take in the disassembly phase are critical to making the rebuild process easier. This story will not replace a manual. You still need to get your torque specs from a manual, and possibly some info on the exact model you are working on.


1)  Never use an impact wrench with an Allen bolt. It just ruins the bolt or engine cover and rounds out. You can use the type of impact driver that you hit with a hammer, but we don’t. A quality Allen wrench or socket that fits snugly will usually work fine.

2) Never use an outside automotive-style puller to pull off the flywheel; buy the correct puller. Some 450s (like the TRX450R) get oil to the connecting rod through the end of the crankshaft. If you use a normal puller, you will destroy the engine.

3) The correct puller has a cap that threads over the crank end to protect the oil passage. Most ignition pullers from Motion Pro are the correct one. Using an outside puller will usually ruin the flywheel.

4) Never use an impact wrench on a countershaft sprocket without holding the sprocket. An impact tool’s ratcheting is very hard on transmission gears and can lead to broken parts. A Motion Pro clutch tool is awesome for holding onto the sprocket.

5) Never try to pry cases apart or the cylinder off using a screwdriver or pry bar. That will always damage the gasket surface. Usually a good tap with a dead-blow hammer, plastic mallet or other soft hammer will break loose the gasket surface, and you can tap it apart. If you are not sure of yourself, Motion Pro offers a case-splitting tool, but they run just over $150; most pro mechanics just tap the motors apart.



6) Always try to put the motor at TDC (top dead center, meaning the piston is at the top and the valves are closed) before you start disassembly. To accomplish that, you must remove the cam cover and the timing covers in the crankcases. In some cases (again, like the TRX450R), there are timing plugs on both sides of the engine. Check the manual to see how you line the marks up to make sure the engine is at TDC.

Make sure you identify the TDC mark on the flywheel. If there is any question, look at the piston (once you remove the head) as you get it to TDC and then verify the mark. If you have it on the fire mark instead of TDC, you may bend an intake valve.

7) After the engine is at TDC, use a feeler gauge to check the clearance for each valve. Make sure that you write down the measurements. That will make it much easier to reassemble the engine later on. If the valve clearance is significantly tight, or if you have had to adjust the clearance twice, you will most likely need to have new valves and springs installed and have the valve seats cut. If you have CT Racing do that job, the head will be returned with the valves all shimmed and the head ready to go back on. That saves a lot of time and headache on reassembly. If you decide to have the head machined locally, look for a company that does performance motorcycle engine work. In our opinion a CNC machine or a dedicated valve machine like Serdi do the best job.

8) The most common feeler gauges you will find are short and straight. Those can be a challenge on some four-valve heads. Longer feeler gauges with an angle bent into them are better, angled ones that taper narrower at the tip are better still, but the Motion Pro ones shown here on the right are our favorite. The center section is stiff and easy to handle and control, and the angled feeler gauge portion is stiff and narrow. They work great. Thinner gauges will eventually fatigue and the tips will break, but they are reasonably priced.

9) You need to have slack in the cam chain before you work with the cams. Make sure you line up the timing marks first, then loosen the cam chain tensioner before you start disassembly. They make special tools to retract the tensioner plunger. In this photo, the mechanic is retracting the tensioner with a thin screwdriver, but it is easier to simply remove the entire tensioner body.

10) Do not try to rotate the engine with the tensioner removed. It is easy for the chain to skip teeth, and that can allow interference between the piston and the valves. You can turn the engine when the cams are out, but pull up on the cam chain while you do. You don’t want to allow the loose chain to bunch up around the crank sprocket. There are little guides that can be damaged or bent.


11) Don’t hesitate to use a heat gun or a propane or MAP gas torch to heat nuts or bolts that have thread-locker like Loctite on them. Even the semi-permanent locker like red Loctite will break loose pretty easily if heated. Make sure that your work area is well-ventilated and free from any flammable liquids or fumes.

12) You don’t need a special tool to break loose the center nut on a clutch basket if you have an impact gun. After folding down the locking tab, leave the clutch plates in place, just put pressure on the top plate, and hit the nut with an impact wrench and the nut will come right loose. If you are removing the nut with hand tools, you will need a clutch tool like the Motion Pro one we used on the countershaft sprocket.

13) Inspect the inner and outer clutch baskets. The grooves in the inner hub/basket and the fingers of the outer basket should be smooth without notches or indentations. The clutch plates need to slide easily on these wear surfaces for the clutch to work correctly.

That is why aftermarket clutch parts are hard-anodized or have steel inserts for the plates to rest on. The pressure plate should be smooth and not discolored where it rests on the top friction plate.

14) Use a sharp single-sided razor blade to clean gasket surfaces. You can hold it by hand or get a handle like this. Take the utmost care to keep the blade at a low enough angle to allow it to slide across the gasket surface rather than dig in or gouge it.

15) Keep the gear shafts altogether. A zip-tie on each end keeps all the gears, shims, and spacers in place to make assembly easier. Make sure all the parts are clean to aid inspection and make sure that you are ready for reassembly. While inspecting the gears, look at the gear teeth, but mostly look at the dogs on the sides of the gears, as well as the slots that they engage. You don’t want to see any wear or rounding.


16) Look at the tips of the shift forks. They should not show any wear or discoloring. The left and center ones look good, but the right one is bad. Transmission bearings should turn smoothly with no rumble or crunchy feeling. As you are cleaning the bearings, most people blow them out with compressed air. Do not allow a clean, dry bearing to spin with the compressed air.

17)  After the engine is taken apart, drop the cam chain on the work surface. Pick it up by one link. If the chain has tight links like this, it is toasted. It is cheap insurance to replace the cam chain at every rebuild.

18) This was a water pump impeller shaft. Now it is junk. The surface needs to be smooth enough for the seals to be liquid-tight, and with the surface ragged like this, it will not keep the water and coolant where it needs to be.

19) As the engine comes apart, take photos with your phone of anything that you think you might need to remember in regards to how it was aligned or assembled. If you see small parts like this pin that will get lost, pull them out and seal them in a sealable snack bag. Label the bag with a permanent marker.

20) You need to make sure all of the parts are cleaned up. We aren’t just talking about grease and dirt. The rust on this case dowel must be cleaned off or, ideally, replaced after the gasket material is removed.



There are other problems that you might face. If you are rebuilding a dual-overhead-cam motor, most of them run at least one end of the cams directly on the aluminum head with no bearings. If the engine was run hot, had dirty oil, or was otherwise mistreated, the head can be damaged. In many cases, a damaged head can be repaired. CT Racing and Millennium Technologies have ways to repair damaged heads.

The one thing on a rebuild you won’t be able to do at home is to hone the cylinder. If you are replacing it with a new cylinder, you are set. Most local shops are not set up to hone Nikasil cylinders. That’s another thing that you may want to send to CT Racing, Millennium Technologies, L.A. Sleeve, or a local engine builder. Most shops just don’t have the workload to support the cost of a diamond hone to correctly service the cylinder. Follow the manual, don’t use questionable parts, and use the correct tools for the job, and you will be surprised at how easy this job is.




Barnett Tool and Engineering: (805) 642-9435,

Clutch components, plates, and springs

CT Racing:, (562) 945-2453

Engine rebuilding, parts, engine mods, 

CNC head service and more.

Cylinder Works: (515) 402-8000,

Cylinders, pistons, and gaskets

DSS Inc.: (208) 634-7292, [email protected]

Duncan Racing: (619) 258-6306,

Engine work, modifications & head work

Harbor Freight Tools:


Hot Cams: (515) 402-8000,

Cams, cam chains, valve springs

L.A. Sleeve: (800) 822-6005,

Cylinder sleeves and repair

Millennium Technologies: (888) 779-6885,

Cylinder repair and plating and cylinder head repair

Motion Pro: (650) 594-9600,


Ricks Motorsport Electrics: (800) 521-0277

Starters, stators, rectifiers and regulators

Sparks Performance Products: (661) 872-4343

Engine building and modifications, head work

Steahly Off-Road: (800) 800-2363,

Ignition pullers, flywheels and clutch parts

Wiseco Performance Products: (800) 321-1364,

Pistons, sleeves, valves, cams, gaskets, clutches, parts and complete engine kits



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