Ed,

I go along with the theory that the problem was because of a steel GP in the aluminum head. There is an electro chemical action that causes an electron flow between dissimilar metals. This causes a solid start diffusion that fuses the 2 metals together. Carbon being soft and brittle does not fuse metals together.

I had the same problem in my 6.3 liter MB gasoline engine that had an aluminum head. It had a spark plug stuck in the head. I stripped the threads removing the spark plug and had to repair the head threads with a helicoil.

The remedy for this is to use a Never Sieze type compound on the threads and every so often loosen and retighten the GP to break any bonds that may start to form. MB certainly didn't make that an easy job on the later engines. Progress has its detractions.

P E H