My home inspection of Speedmaster heads

12many said
If you did find a longer plug the shell threads are going to then protrude on the short side. You want all of the shell making full contact with the head.

From the picture in 610 the plug needs to be about .060 deeper at the most.

You are correct that you sone want any part of the shell in the chamber. The face of the shell should be flush with the chamber.