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.