Top End Rebuild
Rebuilding a top end of a 2 stroke dirt bike was a lot easier than I thought. I'll admint that I was pretty intimidated before I got started but after digging in it wasn't too scarey. I always take pictures when I do a bigger project like this one as a way to map out what i have done and more importantly, what it should look like when I'm finished. I have broken the process down into these easy steps.
1. Clean your machine, you are about to do open heart surgery and the least little microscopic piece of dirt can have your baby back on the cutting board in no time.
2. Strip it down, take off all the plastic, seat, tank, pipe, sub-frame. Take off everything taht is going to be in your way, it is not worth the hassel of working around something that can easily be taken off. Don't forget to drain your radiators.
3. Remove the bracket that bolts the "jug" to the frame. Now that you have the "jug" stripped of all "veins" now you can start pulling it apart. Take out the spark plug and remove the bolts that hold on the head of the jug, there should be five or six of them. Then you will be able to see the top of your piston.
You also should be sure to disconnect the power band by taking off the side cover and disconnecting the rod that goes from the top end to the bottom end, as shown above.
4. Now the fun part, remove the four bolts that hold the jug to the case, some of these bolts will be hard to get to, you may end up getting creative with some of your tools, don't strip them out or you are in a world of hurt.
Once you get the jug off you will see the piston, as you can see this one is scored up pretty bad, I seized this one up while in 4th gear in the power band full out fist full of throttle, it was a great day until I had to push it back to the truck.
5. Remove the piston by taking out the retainer rings on either side, they are in the holes. There is a spacer that is in there as well, take that out, and try not to lose the retainer rings just in case your new piston didn't come with them. I recommend a Wisco piston, they make a great product.
6. You may want to have your jug bored out if it is damaged by the piston. Either way make sure it is clean and smooth before you start to reassemble it. Install the new piston the same way the old one came out. Be careful when installing the retainer rings.
7. Now you can slide the jug back over the new piston, ever so gently. You will have to hold the rings tight while you slide it over the piston. There are two rings on a 250cc and one ring on a 125cc engine. Watch out for the power band rod. This is always the hardest part of the rebuild for me. Good luck.
8. Now you can start putting everything back together. Don't forget to put coolent back into your machine before you try to kick it over. It is always a good idea to double check each bolt to make sure they are tight and secure before you try to start it as well.