It’s time for a little review, children. Mr. Know-It-All gets far too many queries that are generated by fear.  Nothing, apparently, is quite a terrifying as rebuilding a four-stroke to the average millennial. There is no doubt that the cost of rebuilding your four-stroke engine—be it tired or wounded—has escalated as the machines have gotten more complex. On the plus side, technology has helped to increase their lifespan, especially the larger-bore machines, but their ability to rev and the high-power output of the smaller 250s render them vulnerable to 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.

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 does make the process easier. If you’ve been thinking about tackling a rebuild, we would like to offer you some tips to help you avoid some beginner mistakes. The steps you take in the disassembly phase are critical to making the rebuild process easier. This story will not replace your manual. You will need the manual for torque specs and other information specific to the exact model you are working on.

Most of the kits have what we call the “automatics”—the things you should always change if you have the cases split. That would include the main bearings, seals, cam chain and 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 the engine and transmission bearings.

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. Do not use an outside automotive-style puller to pull off the flywheel. Buy the correct puller. Some 450s get oil to the connecting rod through the end of the crankshaft. If you use a normal puller, you can 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 reasonably priced, so get 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. The 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. Do not 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 lack confidence, Motion Pro offers a case-splitting tool, but it runs just over $150. Most professional mechanics just tap the motors apart.



1. Always 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 this, you must remove the cam cover and the timing covers in the crankcases. In some cases, there are timing plugs on both sides of the engine. Check the manual to see how to 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.

2. 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. 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. Many companies like 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. A CNC machine or a dedicated valve machine like Serdi does the best job.

3. The most common feeler gauges 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 are our favorites. The center section is stiff and easy to handle and control, and the angled feeler-gauge portion is stiff and narrow. These work great. Thinner gauges will eventually fatigue and the tips will break, but they are reasonably priced.

4. 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. There are special tools designed to retract the tensioner plunger. This photo shows the mechanic retracting the tensioner with a thin screwdriver, but it is easier to simply remove the entire tensioner body.

5. Do not try to rotate the engine with the tensioner removed. It is easy for the chain to skip teeth, and that can cause 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.

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

7. 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 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.

8. 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.

9. Use a sharp single-sided razor blade to clean gasket surfaces. You can hold it by hand or get a handle with the blade attached. Take 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.

10. Keep the gear shafts all together. 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 make inspection easier, 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 and the slots that they engage. You don’t want to see any wear or rounding.

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

12. After the engine is apart, drop the cam chain on the work surface. Pick it up by one link. If the chain has tight links, it is toast. It is cheap insurance to replace the cam chain with every rebuild.

13. 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.

14. As you take the engine apart, take photos with your phone of anything that you need to remember so you will have a reference as to how it was aligned or assembled. If there are small parts that will get lost, pull them out and store them in a sealable snack bag. Label the bag with permanent marker.

15. You need to make sure all 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, the dowel should be 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 might be damaged. In many cases, a damaged head can be repaired. CT Racing and Millennium Technologies (are there more?) have ways to repair damaged heads.

The one thing you won’t be able to do at home is 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. 

Remember: Follow the manual. Don’t use questionable parts, and use the correct tools for the job. If you do, you might be surprised at how easy this job is.


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

CT Racing: (562) 945-2453,

Cylinder Works: (515) 402-8000,

Harbor Freight Tools:


Hot Cams: (515) 402-8000,

L.A. Sleeve: (800) 822-6005,

Millennium Technologies: (888) 779-6885,

Motion Pro: (650) 594-9600,


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

Wiseco Performance Products:
(800) 321-1364,


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