I have read (but never tried) that diesel and acetone in a 50/ 50 mix is the stuff to use.
Apparently it is so good it could even get money out of my wallet!
While I'm sure that cocktail will work, I've read the best results penetrant as-tested by more-than one person in a comparison was 50/50% mix of ATF and acetone. Kroil is expensive, but seems to-be the best over-the counter, it's not-cheap. I like and use PB Blaster.
Yes, a propane torch should provide you with all the heat you need. Especially if you get the pair of chain wrenches, below. HFT, cheap at $10 apiece.
Once you soak the joints for a day or two, with the 'good-stuff,' you might try a pair of these from Harbor Freight Tools (HFT, '
Locking Chain Clamp,' below). One wrench might do it, but two together will probably work a 'whole-lot better. Apply them at a 60 degree spacing, and push and pull them together/apart, and I bet that stubborn joint will release. Just clamp the outside one as-far-back as you can position it from the inside pipe. You don't want to-constrict the pipe inside by clamping the chain directly-over it on the outside pipe.
Penetrant applied followed by heat, and then the torque of the chain pliers, should release the pipes. I suggest working outside, you don't want a flash-fire from the penetrating oil hitting the hot piping to ignite anything near-by, including your eyebrows, hair and forearm hair, or clothing. You do have a couple of fire extinguishers in the work area don't you? A 10 lb, ABC dry chemical extinguisher should work, if you need it, and one in-reserve.
https://www.harborfreight.com/locki...MIprHwu-OX6QIVBeXICh2fCAV5EAQYBCABEgLscPD_BwE
These are sold there, too, but I think the metal type will 'bite' better.
https://www.harborfreight.com/rubber-strap-wrench-set-2-pc-69373.html
