A little background. I recently got my 67 fastback back from a buddy I sold it to some 15 years ago. He had the trans and engine “rebuilt” by non-shop individuals. The last time the engine ran was some 8 years ago. He said it ran ruff and the only way to get it to run “smoother” was to advance dizzy and change plug air firing order. Once I got it home I thought it was 180 out. As I tried to manually rotate engine to TDC it would “mechanically lock up” before I could complete two full rotations of the crank. It stops the same spot each time. I have dropped the oil pan to ensur the new oil pump he installed wasn’t loose etc., removed the dizzy, jacked the rear to ensure no restance with trans, loosened each rocker arm thinking of a broken valve may be blocking cylinder. Since the engine will do one rotation but not two I don’t think it’s a broken valve. So what is stopping the engine from rotating two full rotations?