I am trying without luck to loosen a threaded steel pipe joint (1-1/4 dia) in the supply pipe to a radiator in my one-pipe steam system - I want to move the radiator by about a foot so I need to re-run the piping.

Tried already without success:
(a) Large pipe wrench with pipe to extend handle
(b) Liquid wrench on threads
(c) Warming joint (by running boiler)

Don't want to try:
Heating joint with a flame (too near lots of flammable wood)

 

You need two wrenches. For 1-1/4 inch pipe a minimum of 24 inch wrenches and even then you probably need an 18 to 24 inch extender. Understand that while people use handle extenders all the time it is NOT acceptable to the wrench manufacturer and they will not honor any guarantee on the wrench if you break it. Also, if the wrench breaks you could end up with some nasty injuries.

Anyway, what you first want to do is to hammer the fitting. If you have steel fittings this can be done without much worry. If you have cast iron fittings you need to exercise a bit of restraint. Take two fairly heavy hammers and hold one solidly against the back side of the fitting at the threaded end you wish to remove. Using the other hammer hit the fitting opposite of the steady hammer smartly. Again, if the fittings are cast iron you need to use some restraint or you will break the fitting (which may be okay if you want to remove the fitting).

Do this hammering at several places around the circumference of the threaded portion and you will most likely be able to unscrew the pipe with relative ease. Do NOT hit so hard that you end up misshaping the fitting.

When using the pipe wrench be sure to adjust it so that the inside of the jaw barely touches the pipe. If you leave a space between the pipe and the jaw there is a high probability that you will crush the pipe into an oval shape.

 

Furd's advice is right on the money & sounds like it comes from the voice of experience. The only thing I might add is to try a penetrant called PB Blaster. It stinks to high heaven but is good stuff. Apply, hammer, let soak, apply again, & try your wrenches.

 

Experience? Yeah, a bit.

I sometimes volunteer at a 100 year old steam-electric generating station that has been opened as a museum. Getting some of those pipes apart can be a challenge. Funny thing is that some of those pipes are the same ones I made up when I worked in the plant (for pay) over thirty years ago.