[DeTomaso] NPC- Mustang plug problem

[JDeRyke at aol.com]

There are -or used to be- steel inserts available from K & N and others to 
fix such things. You tap the sparkplug hole oversize with heavy grease on 
the tap to catch the chips (and hope you get most of them!), screw an insert 
onto a spark plug with anti-sieze on the inside and Lock-tite on the outside, 
screw the assembly in and wait overnight. The biggest problem is having the 
insert grab onto the plug instead of the head when you go to replace plugs 
again. In my experience, Helicoils didn't work as well as threaded inserts. 
Sometimes, the wire end hung into the chamber a tiny bit and caused 
pre-ignition. 
On cost- I don't remember- I haven't stripped a plug in any engine in 
decades. The last one was a junk 2-stroke motorcycle I refurbished, and I made an 
aluminum insert with a tapered pipe thread for that one. Simple fix: I 
screwed the aluminum piece into the aluminum head dry, from the chamber side, 
with a giant wrench until it squeeked... Good luck- J Deryke