Changing the piston, AKA top end, on your modern four stroke motocross bike is easier than you think. With the right tools and a bit of know how (thanks to Gregg Albertson) you’ll be able to get the job done in your garage at home! While there’s no hard answer as to when you should replace your tired piston and rings, being proactive can save you from major engine damage in the future. Before you start any repair, always take a look at your manual and follow the steps provided by the manufacturer. This not only will help guide you, but will give you the necessary torque specs and information on special tools if required.
1. The first step is to drain the engine oil and coolant. Once you’ve done that, you can begin to start tear down.
2. To perform any repair or maintenance on your engine, you need to first have access to the engine. Start by removing the seat, gas tank, sub frame/air box, shock, carb/throttle body, engine mounts, radiator hoses and anything else that might be in the way. You’ll also want to move or reposition any electrical couplers/wires that are in the way as well. Removing the engine from the frame is not necessary, however you can remove the entire engine (as shown) fairly easily by removing the swing arm pivot bolt and engine mounting bolts.
3. With full access to the engine, remove the timing plugs on the ignition case, cam chain tensioner cover and valve cover.
4. Our next step is to find top dead center (always find top dead center before any engine maintenance). Your manual will tell you where the timing marks on both the cam and fly wheel need to be, to indicate top dead center. With a socket wrench, spin the crank (through the large timing hole on the ignition cover) through the stroke until the timing marks on both the cams and the fly wheel (seen through the smaller timing hole) align in the proper position.
5. Check the valve clearance, and make sure they are to spec. If not, you will have to adjust the valves to the clearance dictated in your manual upon reassembly. If your valves have proper clearance, remove the valve cap bolts (in a crisscross pattern), and loosen the cam chain tensioner.
6. Remove cam cap bolts and the caps themselves. You can let the cam chain slide down the head.
7. Loosen and remove the outer head and cylinder bolts.
8. Loosen and remove the head bolts in a crisscross pattern (these are usually fairly snug). Remove the head from the cylinder by jiggling it a bit. Next, remove the cylinder from the crank case. Helpful hint, place a rag or paper towel around the rod, to prevent dirt, parts or contaminates from getting down in your case.
9. If it’s time to replace your piston, it’s time to replace your cam chain. Cam chains wear out and stretch like any other chain, and are inexpensive and easy to replace.
10. When changing your cam chain, it will require special tools to remove the fly wheel. Check your manual for the correct fly wheel holder and fly wheel remover. When you have the tools, remove the ignition cover, and use your fly wheel holder to remove the fly wheel nut. When the nut is off, thread the fly wheel puller in the fly wheel, and proceed to pull it off the crank.
11. Once the fly wheel is removed, remove the cam chain from the cam chain gear, and replace with a new cam chain. Reinstall the fly wheel, making sure the woodruff key is lined up. Using your fly wheel holder, reinstall the fly wheel nut, and torque to spec.
12. Now it’s time to replace the piston. Start by removing the circlip from one end of the piston using a small pick or flat head screw driver. Once the circlip is removed, push the wrist pin through that same end, and lift the piston off the rod.
13. Install the new circlip and rings on your new piston at your work station. The directions included with your new piston, or in your manual, will indicate where your ring gaps should face.
14. Once the circlip is secure and in place, drop the new piston on the rod and slide your new wrist pin through. Helpful hint, lube your wrist pin with engine oil before sliding it through. Once it’s all the way through, install the opposite circlip in the piston (be careful not to drop the circlip into the engine, this can be a tricky part to install).
15. With you piston installed, your next step is to install your new base gasket and slide your cylinder on, while compressing the rings with your opposite hand. When reinstalling your cylinder, it’s helpful to lube the cylinder walls with engine oil. Once the cylinder is completely installed, install your new head gasket and head.
16. Reinstall the head bolts, tightening in a crisscross pattern. These bolts typically call for a very tight torque spec, so make sure you tighten the head down evenly. Reinstall the outer cylinder and head bolts (torque to spec).
17. Reinstall the plastic cam chain guide, and fish the cam chain out with an allen key/spring puller/magnet and use a screw driver or T-handle to keep the chain from dropping back down.
18. Double check your piston is still at top dead center (using fly wheel timing marks, or by sliding a screw driver down the spark plug hole and watching for its peak height position as you turn the crank.)
19. Now that you are at top dead center again, install the cams one at a time. The most important thing to do here, is make sure the timing marks on the cams are in proper position (typically horizontal with the head). Since the cam chain is still lose, you can lift the chain off the cam, and rotate each cam into proper position. Helpful hint, lube the cams and channels with cam grease or assembly lube, this area gets the least amount of oil, so lube is your best friend.
20. With your cams in place and in the proper timing positioning, tighten the cam chain adjuster (most bikes will have an auto-tensioner, so simply back the screw out). With the cam chain now tight, reinstall the cam caps and torque the cap bolts in a crisscross pattern.
21. Double check the valve clearance, and adjust if needed.
22. Place the valve cover gasket back on the head, and reinstall the valve cover, making sure everything is aligned. Next, reinstall the valve cover bolts.
23. With your top end completely installed, reinstall the remaining components back on the bike, paying special attention to your electrical routing and couplers. Replenish your coolant and engine oil, and your job is now complete!
Things to Keep In Mind: This article and video is just a guide, not all install processes or steps are the same for every bike. Always consult your manual for specific steps. Keep your manual handy throughout the process, as it will have the torque specs for every bolt (do NOT forget to torque every bolt to manufacturer’s specs). It’s a good idea to use anti-seize or thread-locker on all bolts as well. Another good idea, is to make sure you have all your parts and tools before starting the process. Keep a camera or cellphone handy and take a ton of pictures during tear down, so you have a reference when reinstalling everything. Pay special attention to any dowels between the head and cylinder, and cylinder and bottom case. These will need to be in place when reinstalling. Clean the mating surface of the cylinder and head, removing any of the old gasket to ensure a proper seal. Proper break-in procedure is important to help seat the new rings and piston. Follow your manuals break-in procedure.