I've received a few questions on how I fixed my glow plug problem, so here's a recap and update with some more pictures.
In short I removed the broken glow plug , cleaned out the carbon, chased the threads, installed a helicoil, and trued the seat.
Got the car back together and haven't thought twice about it.. at least 6,000 miles now.
Here's some more details on the problem and the solution.
The glow plug was twisted off.. The head had been removed from the engine/car and the mechanic was afraid to try anything, so I picked up the head and started by...
First yanked the hex part off the broken plug, then drilled out the softer core a little.
Tried a spiral type easy-out that typically works well, not this time though. It expanded the plug some, would have been better off not to try that.
Next welded some 1/4-20 nuts on (one at a time) and tried backing them off with a socket. That didn't work, but didn't hurt anything.
Finally just welded a piece of 3/8" rod to it, then with vice-grips it did come out just fine.
Usually the welded nut trick works, at least on other stuff, this time welding a rod to it worked...
With the glow plug out the threads were still there but pulled and misshaped some.
I cleaned all 4 glow plug holes and threads with appropriately sized bottle brushes and some brake cleaner.
Chased all 4 holes with a tap (M10-1), which I was able to get pretty quickly from my Snap-On guy.
The troublesome glow plug hole I continued to cut the threads most of the way down to the seat. This gives the recoil two stage tap a place to go without having to cut new threads with the first stage.
Then used Recoil kit 38105 which I found new on ebay cheaper than anywhere else. (Franko6 probably has them cheaper yet, but I diden't know at the time.) Tapped only deep enough for the insert.
Put some red loctite on the insert and installed it, then broke off the tang.
Test fit, removed and wiped the extra off a glow plug before the loctite setup.
The next step was to true the seat, which is probably what caused the problem to start with, carbon getting by a bad seat and locking the threads.
for this I used a 3/8" 60 degree countersink, same angle as the nose on the glow plug.
The OD of the countersink needs to be ground down some (I forget how much) so it fits in the hole. I did this with a 5C Colet spinner and a surface grinder.
Next you need a guide to center the countersink aka "seat reamer" to the threads. for this take a glow plug and trim both ends of it and ream the bore to match the shank on the countersink.
Also made a handle to be able to spin the "reamer".
This worked quite well, use the threaded guide to very gradually advance the reamer and just skim the seat enough to clean up all the way around, removing and checking frequently. I removed about 0.008" to clean up.
Here are some picture of the tools I bought, made, and used.
Grigg