My GT5-a has had no problem, neither has either of my GT5's. The main difference between the 48cc and 66cc is power, however they use the same crank (actually the whole bottom end is the same to my knowledge). As far as I can tell, the crank is balanced for the 48cc, so the heavier 66cc piston actually unbalances it. These motors are vibration governed meaning the 48cc then logically has a higher top speed, the question of if it has the power to reach that speed is an entirely different one (a strong gust of wind will take my 66cc from 56kph to 45kph easily). this can be solved with simple mods, everything from porting to aftermarket parts will net you a big gain, however a 66cc crank can be balanced by cutting the piston or drilling the crank (check out crank balancing in the high performance forum). So AFTER the modifications, the 66cc would have more power. there is no replacement for displacement.