Size R drill bit, and 1/8"-27 NPT tap. We drill and tap them with the truck running. We also hold the shop vac near the hole to get as many shavings from the outside we can. Or you can pull the manifold, it takes about 15 minutes.
Okay I'm guessing you've never tapped anything before. So that being said listen up.
DO NOT USE A DRILL TO TAP. I think an 11/32 drill bit works fine, I used one because I couldn't find a R drill bit. Drill out the hole with what ever bit. tap it by hand, its hard to get started but just keep all the downward pressure on it as you can and go slowly, eventually it will grab and start to dig in. Go in about 1.5 turns then back it out, then go in about 2 turns from where you are now and back it. Just continue to tap and back it out, then tap, then back etc. This keeps the threads from bugering up, and also keeps the heat out of it.
You could rub vasoline on the tap, that will allow something for the metal shavings to stick too. I don't think you need any type of lubercant though on cast iron so its just for catching the shavings mainly.
Use one of those telescopic magnets when your done.
i have done mine always while running and put some grease on the tap, as stated drill it with a drill, but tap by hand with a t-handle, and make sure to wear safety goggles becuase with it running it deffinetly blows all the shavings right out.
also put toward the rear of the passenger side manifold, there is a semi-flat spot there that works well. just pull out the inner fender well, get your rollie seat and take your time so you don't break the bit or the tap off.
Yea you will drill a hole SMALLER than the tap size. Then tap the threads by hand with the correct size tap. My EGT probe was a 1/4 pipe thread, so it was a fairly large hole. I took the manifold off, but I wasn't in a hurry. If you have any questions on tap and bit size, take your fitting for the probe to a hardware store. They can get you set up.
There appear to be a lot of theories on how to do this. Having the truck running seems to make sense. Also start wil a very small drill and work your way up. This way you produce very small metal chips that are more like dust. Chips that make there way to the turbo can't hurt it.
Other tips. Keep the drill perpendicular to the surface you are drilling. Do the same with the tap. Take your time.
Just a suggestion. If this is your first time using a tap, Maybe it would be a good Idea to run down to the hardware store and get a piece of flat stock to drill and tap a few times to get the hang of it before diving into the manifold.