Ok.
So I get my Autometer Phantom EGT gauge in the mail the other day, and the directions are doing nothing to help me with the install. The directions say to drill a 1/4" hole and tap that with a 1/8" tap, which makes no sense to me at all. How are you supposed to tap a 1/4" hole with a 1/8" tap? What I did was measured the probe, and nut. The tip of the probe is 1/8", the center of the probe is 1/4" and the nut to screw all of this onto the exhaust manifold is 3/8". Im not sure if the directions are wrong, or if Im reading them wrong?
My second question is, do I drill a 1/4" hole or 5/16" hole to tap a 3/8" thread?

EGT probes are 1/8 NPT aka 1/8 pipes which is much more larger than 1/8 of an inch

When I have to do one I drill 11/32 and than use a 1/8 NPT tap

Alain

That is correct ^^^. You can get a 1/8 NPT tap at any local hardward store. It's fairly large tap. Just drill the hole in your manifold, best to do this off the car. If the manifold is on the car grease the drill well so it "catches" the metal shavings and make sure you vacuum it out good. Then tap it out slowly making sure you get good clean threads. Then just thread in the probe, tighten down the brass looking lock nut and run the wires through the fire wall into your gauge. I was confused when I first looked at mine too, but it's not the big of a deal. It's just like sticking a temp. probe in a juicy steak .... ok, well not exactly, but close :)