I'm in the middle of a top end rebuild on my nephew's bike. According to the Yamaha service manual, there should be an "A,B, C or D" on the cylinder so I can order the right piston. I don't see it anywhere.
It may very well not be there at all. There wasn't one on my 2001 YZ 125. I would recommend taking the cylinder to a dealer or machinist and have them measure the bore to get the correct piston size (unless you have a micrometer, but then you wouldn't be asking).
I'm in the middle of a top end rebuild on my nephew's bike. According to the Yamaha service manual, there should be an "A,B, C or D" on the cylinder so I can order the right piston. I don't see it anywhere.