What size piston?


Oct 4, 2003
i have a 97yz125 on its stock top end, it runs great and has good compression. I found a deal on a piston kit w/ everything but the wristpin bearing. its for the stock bore, and dosnt have much time left on the auction. If my bike is running good now how much of a risk is it to get the std piston before i take it apart? Like if it still runs good now is there any possibility that my cylinder is out of spec? Wait, its plated. If it was out of spec id be screwed anyhow right? Like i would need a replate so the stock size piston would work no matter what?


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Jun 29, 2003
I f the piston is a stock Yamaha it will be either a A, B, C, or D piston. The letter is stamped on your cylinder. If it is aftermarket it will be "one size fits all". I have a 97 YZ125 and did a top end every year since 1997 and the cylinder looked great. I used wiesco pistons. I measured the bore and it spec'd good each time. Last year I sent the cylinder to Eric Gorr to be replated for the first time. Get a service manual if you plan to work on that bike. I will save you lots of time trying to figure out the little things. If you need more help just ask.