Stripped spark plug hole

Here is a great guide on how to put a helicoil to repair the threads.

 

How bad is the damage?

Threads can be reformed in-place by running a tap in the hole, but this won't replace missing metal.
If the damage is minimal you might be OK.
Thread lock might help. Maybe someone else can comment on this idea.

If the damage is worse you'll be looking at replacing the damaged and missing threads entirely with a threaded insert. Helicoils and time certs are the most common.
If you have to go this path I'd recommend using helicoils. They involve drilling and tapping a smaller hole than time certs require. If the helicoil somehow gets screwed up too you could still move up to a time cert insert, but can't go down from a botched time cert to a helicoil.

Kits for both are available for spark plug threads. Not all kits include the necessary drill bit.
Because the spark plug is a pretty critical part of the engine you might want to take the head to a machine shop and get them to install the insert, unless you have a good drill press.
It can certainly be done by hand but I wouldn't want my first helicoil job to be the spark plug holes.
My exhaust studs don't quite line up properly after drilling those for helicoil with even a crappy drill press.

 

Yeah, a helicoil will fix it quick. You can get them made for spark plugs at auto zone, advanced auto or other auto stores really cheap.

 

Make sure the seat isn't cracked or missing a piece. If the plug won't seat, regardless of a heli coil, you'll be SOL.

Use a little red loc-tite on the heli coil when you install it for some added strength. Just don't loc-tite the spark plug lol.

 

Another option from the Heil-coil repair is this other type that is more of a solid bushing type.

http://www.oreillyauto.com/site/c/d...mn=Performance+Tool&mc=PFM&pt=N2377&ppt=C1986

 

Awesome.