There are kits for this called "Heli-Coils". Auto parts stores have them with the correct size drill bit, tap, and about a half dozen inserts. You drill the hole, then use the tap to make new threads. The insert is a stainless steel spring that you wind in with the supplied plastic tool. Its outer threads match what you just tapped and the inner threads match those on the spark plug.
When doing this with the head on the engine you have to take special care to not allow metal chips to fall into the cylinder. There's a real good chance they're going to blow out when you start the engine, but to be safe, put a bunch of grease on a small piece of rag and stuff that into the cylinder through the spark plug hole. Don't use a paper towel because that will just shred when you try to pull it out. When you're done drilling and tapping, use a vacuum cleaner to suck out as many chips as possible, then hope the rest stick to the grease on the rag. Use a needle nose pliers to pull the rag out.
Watch how far you wind the insert in too. If you go a little too far you'll just have fewer threads holding it in place. If you don't go in far enough the last loop or two will prevent the spark plug from going in all the way. They need to seal against the machined surface on the head. They will not seal properly against the insert. Once you break off the tab used for twisting the insert into place, you won't be able to go further with it. All you can hope to do is catch the end with a needle nose pliers and unwind it to take it out, then try again with another insert.
Saturday, October 27th, 2012 AT 1:56 AM