Ebeneezer Geezer wrote:
Maybe this is, but I do not know the answer to it.

In order to remove a bathtub to replace it, you have to undo the
drain.

Did this job last week. Tried to unscrew drain as it went in; it simply
broke off the pipe leading to the bottom of the drain when turned. Ended up
having to painstakingly cut it out with a dremel disk cutter. YMMV.