interesting

[Rm250RF900R]

Well I decided to tear into my it200 today. It has been sitting for about 8 months due to a broken kick start shaft. Well, I took off the top end and found a nice sized "hole" if you will. Its not really deep, but its definatly noticeable. It still has good compression. It was running fine then it stopped running one day. Then the kick shaft broke. But now I know why it wasnt running. The woodruff key on the flywheel sheared. Before this happened it was running really good. So I dont think this hole had anything to do with it not running. It was just the stupid flywheel key. So, what do you think I should do about this? New piston, rings, and a hone? I bought this bike strictly for a beater. I bought it for $250. I dont want to put a lot of money in it. I already have about $200 in the engine. If I dont need to rebuild the top end then I wont. My Rm250 on the other hand. Is not a beater.

P.s. Sorry for the bad picture of the piston. But you should get the idea.

[GPracer2500]

Pic is blurry so it's hard to be sure....but judging from the location of the "hole": When your woodruff key sheered the ignition was dangerously advanced before it quit running. Way too much ignition advance is essentially the same thing as the abnormal combustion phenomenon known as pre-ignition. The common failure mode with a pre-ignition condition is a hole in the center of the piston. Luckily the engine stopped before it was able to go all the way through and send metal bits into the crankcase.

With a new piston, rings, hone, and woodruff key it sounds like you'll be back in business.

[Rm250RF900R]

Yeah It didnt go all the way through. Its just kinda on the surface. Bad part is most of the part number is gone on the piston. I can only see D00A6. Then below that theres a totally different number. I wonder if the piston is stock or it got replaced sometime over the last 22 years? Can any one help me out? I need to know what piston I need to replace this one with.

[Rm250RF900R]

Well I figured out the piston I need. I need piston 522MO6800. But according to wiseco my bike is on the last overbore (there are no more pistons listed after 68.00). My cylinder is only scorched. So If I only hone it do you think I could get away with just ordering another 68.00 mm piston? Or do I have to resleeve it and all of that? Thanks.