[Bob Phillips]

Thanks Hanno for confirming that early R975s had high compression pistons for use with higher octane fuels. Twenty years ago when I was actively buying R975 parts one item that every big dealer had was quantities of the modified pistons ( # 202050 I think) these were the downgrades to 5.7 compression ratio for use in the early C1 engines. Unlike aircraft pistons ( and later tank pistons) these downgrades were externally very similar in dimension to the aircraft piston and achieved the lower compression ratio by creating a concave centre in the piston hence a lower compression ratio. Unlike the other pistons it was of cast ( not forged) construction and did not have the heavy reinforcing grid/baffles in the underside of the cylinder head. I remember being told by one of the " old timers " that they were " no dam good " because the cast pistons tended to crack early in their life span and were soon replaced by another 5.7 ratio piston in which the piston pin was moved slighly closer to the top of the piston, lowering the comp ratio but also being of forged manufacture with reinforcing grids inside. There were also apparently oversize pistons available ( I have a cast one in .010 os, and several rings sets in .020) but I never have seen or heard much about them aside from the few examples I have.