[mgarblik]

Thank you for explaining the situation again more completely. I think I understand better now. My suggestion from here which would be fairly easy, is to get a good used flywheel from a 1965-1972 same displacement engine and put it on the engine with a pressure plate that has been zero balanced. IMO, that's your best chance of getting back to what you had 14 years ago when everything worked properly. The 1975 flywheel may have been matched to the lighter crankshaft Pontiac was just phasing in at that time. Things get a little confusing in that time period. Pontiac narrowed the counterweights a little in that period and the crankshafts were a little lighter. They also changed the pilot diameter on the crankshaft/flywheel in that time period. If there was some way to verify the crank was balanced with the old 1965 flywheel and harmonic balancer and these parts were NOT modified to get the proper balance, a replacement used flywheel from that vintage should be close enough to operate properly. The complicated issue is from the factory, the final precision balance was achieved after the engine was assembled and the flywheel was installed. The factory would spin the rotating assembly at a low speed and take measurements. Then a final balance would be achieved by removing a little extra metal from those giant divots in the flywheel. So essentially the born with flywheel was matched and mated to the crankshaft. Made for a smooth engine but caused some trouble in service later when parts needed to be replaced. But it was only a finish balance. The small mismatch should not cause terrible vibration.