back when I dabbled a little (played at..) Formula 1 racing, we cleaned up the threads on the 1/4-32 glow plugs before we used them. Changing a plug every flight, the heads would wear out prematurely if you didn't.

I had an adjustable die that you could open or close slightly with a screw - it was set to just barely clean up the threads a little to reduce wear on the head.