Sounds like a broken ring land, 2nd ring to be exact. Had the same thing happen to me before on my 1970 Buick 455. The cause was to much timing, too much compression, too low octane. The ring land between the second and top piston ring was broken in three pieces. Didn't cause any damage. You can check for this without disasembling the engine by doing a simple cranking compression test. Also you will see some puffs of blow by out the valve oil fill tube. I hope that is not your problem, it'd better if it just a flexplate. One question, How hard you been driving it?
Dean
