The NPT insert has a shallow flange on the top of it There is a notch cut into it. The set screw I mentioned is screwed into the head next to the insert and keeps the insert from unscrewing by it's presence in the notch in the flange. I believe, without the setscrew, the insert may unscrew if you unscrew a tight glow plug. Both the set screw and insert are steel. Mentioning that they are dissimilar metals wouldn't matter since glow plugs and head bolts are steel also and are screwed into the aluminum head.

I'm going to try the hi-temp silicone just because I have some left over from making glow gaskets in the past. I have never seen that stuff fail using it for back plate, exhaust or carby gaskets. What do I have to lose at this point?