I have a ~67cc China girl from boygofast with about 1000 miles on. I did a tear down because I knew at least one bearing was bad, turns out both are bad. I'm also doing some port work while I have it apart. I measured the inside and outside diameters, and they look to be about 15mm and 35mm. I really don't know much about bearings and which type would last at least 3000 miles.