Some additional information....motor is 25 years old, either the riser or the exhaust manifold (or both) are bad/leaking, allowing water into the piston, compression of water knocked out the plug, upon exit/down stroke hung up valve. BTW, the electrode was found after pulling starboard exhaust. And really, no evidence of damage. To internals. And since I wasn't there for the machining rebuild process I cannot comment about any damage/pitting to the head/seat of that exhaust port.