On Sun, 16 Mar 2008 12:22:30 -0700 (PDT), Carl M
wrote:

I have a 3/4" pipe thread hole in an aluminum plate, and I need
to be able to connect to both sides of it using pipe fittings, with
the through bore size not so critical. I have been looking around the
web trying to find out what to call a double sided reducing bushing,
but so far no luck. Short of having one made, I'm wondering if anyone
has had this problem before and what you did about it. FWIW, the
fittings would hopefully be stainless, but there's no serious need for
it other than everything else in the system is.
Thanks!

I would take a 3/4 to 1/2 reducer, bore it out so a 1/2 nipple will
pass thru, silverbraze or weld the nipple in place. That gives you 1/2
NPT male threads on both sides of the plate.