2 years ago I bought a $120 66cc kit on ebay and it had 40mm. I sold it to my neighbor and the engine is still running. Only thing we did was grease the gears and install new clutch pads and then loctited the studs it has over 1200 miles though and hes on his second bike for it (the motor outlasted the huffy frame) now hes running it on a mountain bike and hes using the smaller version of the high compression head and a racing carb and boost bottle.