Bill Chernoff wrote:

When you say #18 screw holes I ssume you mean #18 drill. Are the screw
threads 6-32?
4-40?

Yep, #18 drill, which is a little larger than even a 10-32 thread. Here
is a very very handy reference of drill sizes, including which size you
drill before tapping which thread, that I bookmarked long ago:

http://bobmay.astronomy.net/misc/drillchart.htm

Google is my friend.

By the way, the numbers Bingelis recommends are 2 9/32" (vs 2 5/16") for
the instrument, 0.169 (vs #18, 0.1695) for the screws, and the same
spacing.

I think you can drill the screw holes any size you wish (which is
probably what I will do), #18 is what is specified in my build manual.
I think it's well oversize and leaves A LOT of room for error. It's
almost (not quite) big enough for the head of a 6-32 screw to fall through.

I'm not sure where a regular person finds a #18 bit. Aircraft Spruce
doesn't list them. Ace Hardware lists them for about three bucks, I
don't know if the stores normally stock them. Home Depot Aerospace does
not. I might use the next smallest size, and after cleaning up the
burrs with a countersink it will probably turn out close enough.

For screw sizes, umm, well, the 3 1/8 turn needle kit I got from
Aircraft Spruce came with brass 6-32 screws. Specifically, MS35214-29,
which are 6-32 x 3/4. I just checked the fit, and I don't think they'll
be quite long enough, so I'll probably order several -31 (1" long) when
I order hardware for all my other instruments.

Bingelis recommends 6-32 brass screws, adding that 8-32 while acceptable
are unnecessary.

The only instruments I have that came tapped are Westach EGT/CHT gauges,
and these are tapped for 6-32 (definitely not 8-32 or 4-40).

Last random thought, I wonder what they do in parts of the world that
have left the dark ages and embraced metric