I think I used 1/4-20 but I need to go look later for sure. I will tell you to be careful doing the tap work. That cast iron turns to crapola when it’s been heat cycled millions of times and it will grab the tap, snap it off and poo poo all over your day.
Get a GOOD cutting oil, not rapid tap or some crap like that, preferably lard based and go slow, keeping the cutting fluid in there and turn the tap backwards every 1/4-1/2 turn to break the chip.
Old, nasty, heat cycled cast iron is not fun to play with.
