nrp wrote:

From a mechanical engineering standpoint (and not necessarily
aircraft!)

Studs are generally used so that normal disassembly doesn't wear on a
non-replaceable soft part (i.e. the cyl head). Studs are stronger for
that reason.
I figured that was the reason. However, these studs corrode instantly
and make future removal difficult or impossible. So it would seem this
method is not accomplishing that. Many use various penetrating oils to
attempt to break the corroded nuts free. Although once you get the nut
off, you may see that the stud has corroded sufficiently that it is
unusable anyway. I have used a cutoff wheel on a Dremel to cut the
siezed nut off a couple of times.

..stuff snipped
Have you tried using high temerature antisieze paste?
. more stuff snipped
Never tried anti-sieze because I worry that the nuts would work loose.
Never talked to an aircraft wrench who used it and/or reported success.

.still more stuff snipped

Helicoils are effective for getting good strong threads in soft
materials, but once you wreck it, there is no additional fix without
removing an unacceptable amount of parent material. I suspect that
(and $) is the reason they are not used in exhaust attachments.
Fortunately they do use them effectively in sparkplug holes as you
know.
It is common for cylinder overhaulers to weld all the exhaust holes shut
and then redrill and tap them as part of the overhaul process.

Thanks for the feedback.
Mike