You broke a piece of the top piston ring land off, aka chiped piston, not a term I use because it;s not really a "chip" but more of a "chunk" of aluminum off the top of the piston (directly above top piston ring). First of, 13psi on a stock engine is twice as much as it should have, why do you have that much on a stock pulley? There has to be a problem somewhere.
What happens is the sudden rise in combustion temperature from detonation causes the top piston ring not to be able to transfer the heat to the cyclinder wall fast enough and the ring expands to the point to where it locks together and pulls the top part of the piston off at TDC when the piston starts back down, that is what flattend your spark plug.
It would be a good idea while your rebuilding the motor to open the top ring gap a min. of .010-.015 for saftey.
