Repairing a cross threaded spark plug

If a spark plug is stuck in the cylinder head, and you suspect that it may be cross threaded, a solution to the problem is to spray the offending spark plug liberally with WD40, and ease the plug out a 1/4 turn at a time, tightening back 1/8 turn each time. Once the plug is out, take a used spark plug with a good thread, and coat the thread with "Copperslip".

This "Copperslip" will act as a lubricant while you ease the used spark plug gently in and out of the cylinder head. When you reach a point where the plug can be turned in and out completely using only your hand, it is time to fit a new spark plug, and torque it up to the correct level.

It will take a lot of time and effort to get the old spark plug out, and clean up the thread in the cylinder head, but this solution may allow you to avoid costly head replacement for the time being. (Obviously this all depends on how servere the damage was to begin with).