Hi, I just purchased on of these bikes, needs a piston and ring set, already has new jug, head, etc.... My question is what is the difference other that about $50 in the 1975ish piston, from the 1980 piston? The diameter is the same, the rings look the same, actually they look the same as the ones that came with the bike, however it looks like the 1980 style has 4-stroke type rings... Need to buy my parts, your answers are appreciated! Thank you!!