On my '93 Rm I just brought the whole cylinder and piston, handed it over to the mechanic and he took care of the rest. Turns out my cylinder's already been oversized and replated. So my piston is a 55mm, and no issues whatsoever, I don't personally have any interest for going any biger than that. It's important to get the measurements right cause the piston I pulled out, and I still have it in a drawer, was a stock 54mm.... A full 1mm undersize! And I swear to god the bike ran alright, so who would have known?