A friend of mine has a '91 Ford Probe which runs fine, except it throws a spark plug every once in a while.

The plug (a 14mm thread like ours, but with a slightly longer reach... 0.75") came out once and some mechanic helicoiled the head and reinstalled the plug. Now it's done it again and the car has sat for about a year.

Here's my idea:

Drill out and tap the head for a 18mm threaded spark plug, NGK's website if pretty useful in this regard... but I was wondering if anyone has ever heard of this being done with success?

I don't see why it shouldn't work, as long as the heat range and reach is the same as the stock plug.

If it doesn't work I'd end up buying a $200 to $300 head, a $100 gasket kit, and wasting my time repairing that piece of junk.

Please tell me this isn't insane.

 

You probably don't have enough material left to thread properly for 18mm unless you install another helicoil. However Helicoil also makes an insert called save-a-thread. It is a metal sleeve that screws into the head and is swedged in place. The kit comes with everything you need. When we install them, we put loc-tite on them. Give that a try and be sure when you do, the exhaust valve is open so you can blow the particles out the right direction. Probably won't do the cats any good though.