Well, what exactly is the diameter of the piston? Did someone say above that it's the same as a 66?
Here's an old thread of mine where I asked for the diameter of a 66- the answer I got was 47mm. The 50cc piston is 40mm.
I'm assuming if there is ANY piston difference, that the stroke is still the same length.
An 80 would have to have 30 percent larger piston diameter than the 50c.c. motor?