What klem1 said, plus multiple applications of Blaster PB penetrating catalyst: https://www.amazon.com/Blaster-PB-PENETRATING-Lubricant-OZ/dp/B00XOSYNM6/ref=sr_1_3?crid=2NJFDZQHKAVK&dchild=1&keywords=blaster+penetrating+catalyst&qid=1631776361&sprefix=blaster+%2Caps%2C231&sr=8-3

That prevent's someone screwing another cap on and putting you back where you started. If Coby send's 50% what he paid the plumber to say it couldn't be removed,I'll tell him how it can be removed and new reinstalled for about $30-$40 in material and an hour labor. If Coby live's in N Tx,I'l comel do the job parts and labor for 50% what the inbred snuff drooling plumber quoted. I believe half price is quit generous on my part,anyone care to try and beat it? With the virus issue,I should be charging double for contactless accessments.

So usually that's a brass cap and it's been leaded in - just like the other joints.

About the only way it's coming out is to use a chisel. If you do that properly you can cut the brass enough that you can fold it in on itself and not mess with the lead.

Cutting out a section of pipe and going back with PVC and Fernco couplings isn't a bad solution and may ultimately be faster. It's also a very common solution to the problem.

Jake-

If that's the case, the main concern I would have is how hard it would be, especially for the OP, to cut that joint out because it's in that corner...

I put a centre point tool into my hammer drill After soaking the threaded top of the cover

with wd40 , or similar rust remover , I established four grooves around

The edge of the plate and gave it good hit in these four places with the

hammer drill. Worked for me , and the plate had been untouched for more

than 30 years. Faied to mention I aimed rhe centre point in the anti clockwise direction

And took it steady .

HU:

Pure luck you didn't split the cast iron.

Have added a bit more advice above Joseph. Thanks for your mention.