I am fixing up some 1937 Warren-Webster in wall radiator valves that are leaking. These were converted from steam to hot water years ago. In the process, I came across a badly rusted 3/4" supply nipple that extended down into the floor that I felt needed to be replaced. I was unable to loosen it so I cut it off and dug the nipple end out of the elbow. As I have done before, I used a dremel to cut the inside of the bad nipple so I could collapse it for removal.
Unfortunately, this time one of my cuts went completely through the threads perpendicular, top to bottom and I ruined the fitting. I have searched this and other places on the web and find that attempting twist off the old elbow will certainly only make matters worse.
I found that a product like Harvey's Plumber Epoxy Putty supposedly can be used to repair threads and pipes but see mixed reveiws on how permanent it might be but none addressed my situation specifically. I got some and played around with it by using my dremel skills on a new fitting I made a new ruined fitting to test it. I then applied the epoxy and ran a tap through it once it hardened. It seems OK visually but I would like some expert opinons from those that have used this stuff. Perhaps this is the perfect application as it must be good for something.
Lastly, I have just read a thread here (Steam Pipe Removal) that mentions cracking the old elbow off and replacing it verses trying to twist it off. Perhaps that is a better option but short of using a nut splitter I would have no idea how it is supposed to be done.