RE: Glow Plug thread difference?
Turk:
As W8YE said, all glow plugs, other than glow "Heads" and Nelson plugs, are 1/4-32 UNEF.
You can wait until one of your Fox plugs fails, then saw a couple slots across the threads and use the result as a thread "Chaser" to be sure the threads in the head are clean before you put a new plug in.
Bill.