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I installed a low-level float switch in my header tank today. I had
previously applied a .375" thick by 2" diameter aluminum hard point on the
aft side of the header tank approximately 3" up from the bottom (I forgot
to measure the exact dimension at the hangar today).

I drilled into the hard point starting with a 1/8" bit and working up to
5/8" using a drill press. The trick here is to keep the chips from falling
into the tank. This can be accomplished by pressurizing the tank with a
constant flow of air from your air compressor so that the chips blow UP out
of the hole, not down into it. I just tie-wrapped the valve of my
blow-nozzle and stuck it into the tank through the fill port. I plugged up
the hole around the hose with a rag so that all the air was forced out the
hole I was drilling.

I was unable to find a 16:1 tapered reamer big enough at the hardware
store, so I bought a Black & Decker rotary rasp and used that to enlarge
the 5/8" hole slightly and add the appropriate taper. Sure, it was a
half-a**ed (half-rasped?) way to do it, but it worked OK. Only cost $2.50
for the rasp, too.

Tapping the 1/2" NPT hole was a pain, and took a couple of hours. Again, I
pressurized the tank as before to keep the chips out. It worked well,
because when I flushed the tank after the float-switch was installed, I
found zero chips inside.

I ended up mounting the switch so it is floating with the tank full
(contacts open), and the contacts close at 2 3/4 gallons on falling level,
and reopen at 3 gallons even on rising level.

This will allow me to test the unit prior to flight by inserting a stick
into the tank and pushing the float down to verify the alarm comes on.