HOME and how to join Forum Open Area General Scott topics Piston clearance Re: Piston “Bump” clearence

Roger Moss

Sorry, I would need to measure it, but a couple of bits of info in case they are of use. It is wise to have about 0.030″ minimum clearance assembled with a new gasket that is nipped down but not finally tightened. After final tightening and with the stretch that happens at revs, the piston should just clear the top. It is possible to build an engine that turns over very easily and revs easily on power, but listen for the growl that tells you that the piston is kissing the head on the over run. The rod / piston assembly is at maximum stretch when you close the throttle at revs and there is no pressure above the piston. Second point. Good heads are becoming difficult to find and there are a few good looking short stroke detachable heads around. The profile and dimensions are different and the long stroke piston will hit the top unless a fair amount of re working is done. Another point while I am on. I have had a lot of rods through my hands for reconditioning. The length of rods can vary up to 0.030″ from standard and I find long stroke rods are generally about plus 0.010″ on centre distances. If we are doing a blueprinted engine we use a telescopic dummy con rod to measure the bump clearance, but a normal Scott seems to have a lot of space above the piston at TDC. Not the answer you were looking for, but maybe some help. Kind Regards Roger