DIY Chatroom - DIY Home Improvement Forum - tool for twisting really short threaded nipples from inside

a couple of years ago i took a radiator for stripping and repainting and the dude at the shop had a tool that you would fit as a male inside a very short nipple that goes into the radiator and (un)twist it like that. it was conical shape and the cone would go in until it would fit the diameter of the pipe. because the outside of the nipple was just too little to fit a wrench around. and those nipples are made so that they have flat sides in the cross section, as i can remember, so that this tool fits and locks, sort of like a screwdriver.

i want to but that tool but don't know the name. can anybody think of it?

thanks