What happens if you over rev your engine?

The inertial forces on the engine internals cause the engine to tear itself apart. The piston has to reverse itself twice every revolution, which puts immense strain on the rod and its attachments. The valves are also returned into place with springs, and at a certain point the springs can't close the valves fast enough, and they "float" and may contact the piston

basically, your engine tears itself apart from the inside....