It's time for a new top end again. It's been a year or so since my last one and it was overdue again. It's recommended that you do your top end every 40 hours of run time, but if you're like me it gets put off until you really need it. 40 hours is excessive unless you race in my opinion. I probably went 80 to 100 hours on my last top end. A good rule of thumb is to put a new piston and rings in every year IF you ride a lot. I can tell when the top end is hurting from the slight loss in power, and it won't sound as crisp. When the idle gets weak it can be a good sign it needs a top end as well. I did some research and found these hour meters that record the hours that your engine has run by hooking to the spark plug. I think it would be a good investment to let you know your service intervals. Maybe next time I can catch it before it shows any real signs in poor performance. Doing a top end rebuild is pretty basic. First make sure you get a good quality piston and make sure it comes with a wrist pin, rings, and circlips. I have always gone with wiseco pistons and have been very happy. Once you have the piston kit, wrist pin bearing, and a gasket kit, it's pretty much just disassemble and remove the old parts to replace them with the new. I start by removing the gas tank, then the carb/reeds, drain the radiator fluid etc., then unbolt the parts of the cylinder and pull it off the piston. Once the cylinder is removed it will look like the picture below. At this point you need to get something to pry the circlip out of the groove. A dental pick is the perfect device, but if you have a small flat head screwdriver or similar just pry the circlip out of one side and slide the wrist pin out removing the piston from the connecting rod.
Old versus new
Always get a new wrist pin bearing. They are cheap and you don't want to chance one going bad to save a few bucks, you'll be in there anyways so just slide the old one out and slide the new one in.
Install one circlip, then the wrist pin into the piston and through the bearing, then the other circlip. I just use a pair of needle nose pliers to get the circlips in.
Make sure the arrow on the piston goes towards the front of the bike.
Here are the rings installed. They have to be lined up just right in order for the cylinder to slide back over it. I usually have someone lower the cylinder on while I hold first the top ring then the bottom ring in position. There is a small pin in each groove that lines up with a notch in each ring. It really won't go together any other way. It's also critical at this point that as the cylinder is lowered down that the notch on the power valve line up with the pin on the matching linkage from the transmission. Power valves differ somewhat between models so this step may not need to be done on different bikes. Make sure to tighten the bolts in a crisscross pattern slowly to prevent warping. After everything is assembled the operation of the power valve can be checked by taking of the cap on the side of the cylinder and watching the notch rotate while the engine rpm's increase and decrease.
Always clean out your power valve as part of the top end. If you don't, it will end up sticking and causing poor power characteristics. Mine was starting to stick before I rebuilt it last June. It would usually rev up fine, but would sometimes take a second to get my low end power back. It felt like I had about half power until I blipped the throttle to get the valve to close. Also check your reeds each time to make sure they seal correctly and are not cracked.
That's basically it! Although this was a 2001 CR250, the same basic procedures apply to most 2 strokes. If you still have questions, Here is a Short Video that shows a detail top end on a yamaha 2-stroke bike.