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I've used heli-coils and they work very well. If you have minor thread damage from cross threading or overtightening where you "mashed" the threads, then you could try the same size tap first. If you actually ripped out the threads, then it's probably heli-coil time though, or, in very simple cases, you could just drill and tap it out one size larger and use a bigger bolt.
I take it that the messed up thread is the sparkplug threads inside the head. You can buy a thread chaser and use it while the head is still on the car.
The biggest obstacle (besides getting the darn thing lined up) and (turning it when you finally do get it lined up) is keeping metal outside the combustion chamber.
Don't forget to lower the piston in the cylinder if you do this ~ and use plenty of grease on the thread chaser in hopes that it will catch and hold the metal filings.
Since I have never done this(and luck seams to be against me) perhaps I should have someone ELSE DO this. What would be a reasonable price(seams like 1 hour labor)????
Tom
They work great. I had a striped removable roof bolt on a supra i owned and used a heli-coil on it and it worked great. Just be careful not to over tap. :)