CB,
It is correct to consider the dissimilar metal as an issue, but in this
application, it isn't the major one. This is so because the fastener will
not be subjected to immersion, only wetting. So, in application, the problem
is not so different than on a motocycle case or an aircraft. Stainless in an
anti-seize grease is fine. The bigger issue is the thin wall of the mast.
Rivets are much better and if removing the fastener periodically is an
issue, you should use an aluminum riv-nut riveted in place and then use a
316 screw.
Steve

"CB" wrote in message
ups.com...

I need to tap out a 1/4" hole in my mast to 5/16" 18 (no threads in
there right now) and put a machine screw in there.

I have some options in terms of metal types and there have been a
number of discussions about corrosion and so on with SS in contact
with aluminum and how really useless the isolating compounds are in a
sal****er environment etc etc

So what's the metal of choice? I'm seeing brass, SS, silicon bronze,
zinc plated steel.. what's optimal for the aluminum (non-)interaction?
These other discussions don't have a consistent conclusion. (And no
rivets, this is a small job, I really do need the machine screw).

This is a freshwater boat and has been for a long time. However I
wouldn't rule it out of taking it down the east coast one day.

Thanks for any comments,

-CB