Now that I think more about it, I remember getting the 9:5.1 cc and thinking this cant be right. If I remember correctly the original heads where 62cc and the motor had an 8:5.1 cc at that time. If these heads are 57cc and I'm using a stock gasket... I might just be barely hitting the 62cc mark again. I wish I could find the paperwork so I could have someone double check my math.