C. Lee can't be that bad!

My point was that "cleaning up" the threads on a conventional glowplug is going to lose rpm This is an accepted fact amongst those who treasure
every extra rev of the prop.

If your aim is to preserve the threads in you g;ow engine head then that is a different story.

This may have been important in an era before CNC manufacturing. These days both plug and head are made to exacting tolerances.

I just don't think it's important anymore.