Ok so i did a head gasket swap on my sbc 350, did the valves and when i Installed the distributor and went to start it, it idles for a second very rough and back fired through the carb. So i figured i had it 180'd out. Fixed it and this time when i went to start it, it would crank then a loud grinding sound would happen about when crankshaft made one full rotation. I pulled the wire out from the distributor cap so no power was getting to it and that breaking sound doesnt happen when i crank the engine. Im really paranoid right now. Im afraid to touch anything. What could that be?