A small leak in the head chamber won't cause detonation. If it is leaking it will cause lowered compression. Just the opposite symptom of detonation. It doesn't sound like that is the problem. Just don't keep running the motor while it is detonationg like that. It will destroy the piston. If you have rasied the head clearance already it just sounds like you need to run a number 7 or 8 plug. I believe the MC-Coy M59 translates to roughly a number 6 plug. I believe the MC-Coy M59 is also a non-turbo plug. Make sure when you buy the new plugs they are non turbo 7's or 8's as well.