Jim Carriere wrote:
Just a followup to this thread (timing is everything...), I had some
success using 2-1/4" and 3-1/8" holesaws and match-drilling the
screwholes. This is pretty long...

I started the screwholes with a long 1/8" bit, and with these you want
to be very very very particular about getting it centered, then slightly
enlarged them (9/64 or 5/32 as necessary... depending on how far
off-center my match-drilling was), and finally countersunk the cockpit
side of the hole.

Minor "gotchas" and notes:

Westach 2-1/4" EGT/CHT gauges come with their screw holes pre-tapped,
and a 1/8" bit is a bit too large (it will chew up 6-32 threads).

I just about wore out my fly cutter making one very ugly 2-5/8" hole
(probably too much rpm), hence I decided to try holesaws. The hardware
store holesaws made holes slightly undersize. I had to enlarge some of
my 2-1/4" and 3-1/8" holes using a Dremel sanding drum and about ten
minutes of patience per hole. The 2" automotive instruments required
more minutes of patience.

You can trim/debur/countersink big holes in 0.063" aluminum using a
utility knife and file.

Here are the important points of how I made the big holes:

1) clamp the workpiece to plywood (the plywood will stabilize the 1/4"
bit in the middle of the holesaw, otherwise the holesaw will try to
wander ALL OVER THE PLACE!)
2) drill a 1/8" pilot hole where the center of the instrument will be.
3) use the hole saw at low speed (I have a large two-handed variable
speed/max 850rpm drill but used about half that speed... only had a few
very minor kickbacks)

(I don't have a drill press)

Good advice, Jim. I do have drill press, and it helps to get things
straight. If you can do a composite panel...

Start with 2" thick foam. Cut the hole 1/8" larger than necessary. Use
3 1/4" and 2 3/8" hole saws. Cut the back side of the hole square to
about 1/2" deep.

Coil 2" wide, tape covered strips of aluminum flashing and wrap one end
with pieces of wetted 1" tape. The tapes should be long enough to make
one circle of the instrument. It will take about 6 layers of 8oz tapes,
but you want to build up enough to "close the hole up to the proper
size. You may need to do 3 layers, let it partially cure and do 3 more,
because the coil will only 'push-out' so may wraps at once.

Stick the coil into the hole so that the tape is against the back side
of the panel, help it expand to completely fill the hole by running a
screw driver around the perimeter, and pack the corners that you dug out
with flox.

Glass the back of the panel and let the unit cure.

Pull the coil out from the front, and cut the glass for the instrument
holes (real easy with the cured tapes as a guide). Heat the glass with
a hair dryer if you find it difficult to cut.

Match drill holes into the flox for mounting screws.

Sculpt and glass the front of the foam for a beautiful contoured look
with eyebrow shades for each instrument. Drill a 3/16" hole at a 45
degree angle into the back toward the cured tape, without penetrating
it. You are just trying to clear the foam out of the way. Sink an LED
into the hole for instrument lighting. Sand the coiled tape for more
diffusion of the light.

I'm about halfway done.