As a 35 year career machinist I have tapped hundreds of holes in cast iron. I have never used a tapered bit for any of them. Tapping a 1/2” NPT thread in cast iron is going to take a bit of force. I use a T-handle on the tap to give me more power. You can use a socket and ratchet but the will require more force and is harder to keep it straight. I tap about an 1/8 of a turn at a time, then back it out 1/4 turn to break the edge. About every 1 turn, I back it out completely to clean out the hole and check the thread.
As 2-Ton said, go slow so you don’t go too far. One more word of caution, when you tighten the bushing, it will go in one or two threads more after it’s hand tight before it gets tight.
I also prefer pipe dope over teflon tape. Good luck and keep us post on your results.
Last edited by Phak1; 05/29/2020 1:21 PM.
