Dereck,
I have an aluminum standard head, too. I think the stud thread
change has to do with the fact that fine threads don't have enough
grip in a material softer than iron/steel.
I don't think you really need different spark plugs. The fact that
they don't screw in has to do with thread tolerances. I bought a
Colortune glass spark plug to set my idle mixture, and had the same
problem. The standard plugs went in the head fine, but the Colortune
wouldn't. I tried the Colortune plug in the old iron head, and it
went in fine, too. Since I had already put the head on, and didn't
want to remove it to try tapping out the plug holes, I bought the
appropriate die and ran it over the spark plug threads. It cut just
a bit off the plug threads, but it was enough to run the plug in
without stripping the head threads.
BTW, it has been my experience commercial that taps and dies
(Vermont-American, Ace, etc.) are made with fairly loose tolerances.
I have tapped or retapped many holes which seemed "sloppy" when the
fastener was installed. It may be that the thread tolerance in Al is
a little different to account for thermal expansion/contraction, or
maybe the manufacturer has a really tight set of taps...
--
Chris Attias
Aptos, CA
'64 MGB
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