Cast iron is self lubricating so, no threading fluid is needed. I would also place a magnet on the top end of the tap, to help collect the cast Iron shavings and vacuum out the cavity after tapping.
Tap it so you can turn it in 2-3 turns by hand, then use a pipe sealant and install the plug.
EDIT: added the word no! Thanks klhanson!
Last edited by Phak1; 03/20/2021 7:44 PM.
