[ByronRACE]

Ford had (has) problems with the 4.6 and 5.4 engines spitting out the plugs, along with the threads, particularly on the lightning engines. They sell a threaded insert (helicoil, pretty much) kit that you install in-place without removing the head. Either that same kit, or something like it, could certainly be made to work.

However, if there's enough thread there to avoid having to do this...I'd antisieze the plug, screw it in, snug it up, and run it. If it can't hold, you'll know in short order. If this is a street engine that can go a long time between plug changes, I'd consider putting platinum plugs in it and changing the service interval to something a lot less frequent to avoid disturbing that plug. I've seen 70,000+mi old platinum plugs that showed near zero wear. The thinking here is partially lazyness; but also practicality. In 70K miles, chances are better that the head is coming off for some other reason and you can fix it the more durable way at that time. Why do it twice if it'll hold til then.

Byron