got a 89 moto4, I'm in the process of rebuilding it. but i dunno if the piston is std size or bigger? all it says on the piston is YAMAHA 3GHD0 or 3GH00 and AG-15.

 

contact a yamaha nut or dealer, they might help.

i have a 99 trailblazer 250, rebuilt the top end, i thought i was a stock pistopn in it,
so i ordered a .20 piston and rings, after i got the piston, i learned that the quad
was already bored to .40 over, and couldn't return the .20 piston and rings, i was out $$140.

the moral of the story is make sure you know what piston you need before you order!

 

You can get the measurements of what the stock piston is, then take your piston to the machine shop and they can measure it and tell you if it is the same as stock.