Never, never, NEVER turn the prop backwards more than about 1/4 turn. If you do, you run the oil pump backwards. On a dry sump system, this pulls air back into the system and one of the first places it goes in into the valve lifters. Unlike you grand-dad's old Oldsmobile, there are no lifter noises when this occurs. All that happens is that the valve train is quickly battered into destruction. Such as the valve you see sitting crosswise in this cylinder head. The piston in the back came from the same cylinder.