ORIGINAL: av8tor1977
No sealers are recommended on spark plug threads as it can inhibit heat transfer from the plug into the head with it's cooling fins.
If possible, machine a taper in the head to match the taper on the sealing surface of the glow plug adapter. The taper wouldn't have to be very deep.
AV8TOR
This was for a person who does not have a way to machine a taper. Just use the real narrow tape and do just the first portion of threads so the upper part is still contacting metal to metal. This is just for this one engine...kinda a experiment "if you will" There is good product called Anti-seize that is good for the threads of spark plugs. Capt,n