when I rebuilt my bike last year i had to go .100 over. I never had and issues with overheating or otherwise. matter of fact, i used vitos superstock pistons. bike made 58.93 hp to the rear tires with 20cc domes, cpi inframes, stock carbs, no porting. you need to have a reputable shop look at what you have, and see what thier recomendation is. And is that 64.60 measurement taken from the top? or was that at the bottom? bottom wears way more than the top. and the bigbore stuff is shit in my opinion. stock banshee cylinders have small enuff transfers, you would only be killing the flow in my opinion. And by the way, the only reason I removed my .100 over cylinders was to install my FAST ported cylinders.