Mike,

I think you have triggered off one very good discussion here with your
Aluminum/Threads query.

One thing I forgot to add to my earlier comments about anti-seize compounds, and
an issue that GMBulley, Terry, and others brought to my mind is that  you NEVER,
EVER install a plug into a hot engine, a warm engine, not even a slightly  warm
engine.

OK, there might be an exception but bear with me on this.

Holes, be they sparkplug holes or otherwise, always increase in diameter when
heated.

In the piston-engine part of the aviation world, the cylinders are one piece.
That is, they have threaded steel barrels and threaded aluminum heads. Assembly
is accomplished by super cooling the steel barrel (probably with nitrogen), and
heating the aluminum head, then screwing the two pieces together. Once equalized
in temperature they will never separate.

When a spark plug, especially a cold plug, (and plugs removed from a warm engine
because of their smaller mass will  cool faster than the warm engine they were
removed from), is installed in a warm or hot engine, it is guaranteed to seize
in the plug hole. When you torque the plug, even though the torque gage reads
correct, it will not be the proper torque for hole diameter, and when that
warm/hot hole contracts fully with cooling, you will likely NEVER/EVER get that
plug out again without stripping the threads out of the hole.

There is even the possiblity that the  material of the plug hole could crack as
it contracts around the ungiving mild steel of the plug itself, resulting in a
cracked head. Then you have a real problem.

There are always exceptions, granted. But the principle is that, for the most
part,  heated holes and cold plugs do not a lovely coupling make. Or said
another way, parts that are mismatched in temperature don't make for good
assembly in most cases. It's a recipe for trouble.

Be sure your engine is cold when you work with the plugs. Some do it with the
engine slightly warm, but for plug work it's risky business.

John Rodgers
88 GL Driver