if your crunching plugs then there is a good chance your have damaged the engine...Every time you crunch a plug particles of the coil enter the engine and cause all kinds of havok.... that would explain why you keep killing plugs one after another...... Any time you break a coil on a glowplug you really should take the engine apart and clean it to be sure no debris from te plug is inside the engine..... Even tho the pieces of plug wire have broken free they are still reactive inside the combustion chamber and can cause all sorts of detonation issues, which will cause you to keep killing plug after plug...I suggest taking the head off the engine and inspecting the top of the piston and the underside of the head button...I wouldn't run the engine till you take it apart....