Rebuilding the top end on a 2 stroke dirt bike or ATV sounds like a job for maintenance man. Lots of riders freeze at the word "rebuild." It sounds complicated and intricate but while rebuilding a 2 stroke top end takes some time and a bit of patience, it's not that difficult.
In reality, you don't have to be that super to figure out what's more or less a change of gaskets, some rings and the piston.
Nevertheless, because of the perceived complicated nature, riders tend to wait and wait and wait until a top end is absolutely crucial to ride. Common symptoms of a fried top end include lack of horsepower, engine noise and what's called piston slap. Piston slap is when the piston slaps up against the cylinder wall because of a lack of compression or worn out cylinder. You'll know when it happens - loud and annoying.
Rebuilding the top end should fall under general maintenance like an oil and filter change. Therefore, follow the service manual for manufacturer's recommendation interval. Waiting too long and/or repeatedly waiting until its past due can result in engine damage. Mostly though, you'll just have a poor performing bike and a lousy day of riding.
If you don't have a service manual for your dirt bike, get one. You'll need the specific torque and wear specs for the make and model of your dirt bike. Plus, the service manual guides the process so you don't miss anything important. (Note: This "How To" guide addresses a general 2-stroke top end rebuild for riders familiar with the ins and outs of most maintenance needs. So don't get mad at us for not pointing out every bolt and screw you'll remove during the project.)
You'll want a torque wrench and ratchet set. Clean your bike first - you don't want dirt getting inside the engine casing as you break it down. As for reaching the top end you have several components to remove:
- Drain coolant
- Remove seat and fuel tank - radiators can remain
- Take off the top motor mount
- Remove the head and cylinder
Now perform the top end rebuild:
- Remove old piston
- Inspect cylinder to make sure there is no cylinder scarring
- Measure cylinder
- Put rings on the piston
- Connect new piston to the rod using the pin, needle bearing and circlips (Use assembly lube or premix to coat the piston)
- Slide cylinder over new piston using new gaskets. Torque Cylinder bolts
- Put the head on the cylinder, new gasket as well. Torque head bolts
- Replace radiator, fuel tank and seat
- Add fresh coolant
- Start the bike
Uh-oh. What happens if you find cylinder scarring? If you've never seen it, scarring looks like vertical scratches inside the cylinder. Don't move forward with the top end rebuild. Buy a new cylinder (more expensive option but faster) or have the current one re-plated (takes more time but cheaper alternative). Either way your bike gets some time-off for a week or two depending on what route you take for the fix.
An example of cylinder scarring - Courtesy motorcycleforum.com
Furthermore, the cylinder needs measuring to determine what sized piston fits inside. Even if the cylinder is free of scarring and looks ready to ride you can't simply pop in a new piston and put everything back together. Well, you can and most of the time the new piston probably fits within spec but you'll want to ensure the correct size because...
The cylinder bore expands and hours of the piston's back and forth motion turns the cylinder oblong. The cylinder probably looks just fine but you'll need a bore gauge or caliper to determine the new size. If you don't have one, find a mechanic or buy this caliper and we'll ship it same day.
Once sized, you'll breathe a sigh of relief knowing full-well the new piston fits or if you haven't ordered the top end kit you can request an oversized piston. If sizing shows the cylinder bore way out of spec for the available pistons you'll need to send it out for machine work in the form of plating or re-sleeving. This procedure is a big process and not something done at home unless you have the right tools.
Opting to forego the cylinder sizing and popping in an out of spec piston results in low compression and reduced power. Expect engine failure at some point along with broken internal parts.
Assuming you're ready with the right sized piston, replace the needle bearing, the seals and rings. Put everything back together. Don't forget to add the new coolant. Start your engine and let it warm up. Shut off, cool down then start again. Ride an easy 15 minutes or so, let it cool down. Now you're ready to rip.
Written By: AndrewT