110-year old house, 13 solid water radiators with 1 1/2", 2" and probably 4" inch black pipe for parallel supply and return. I decided to tear out all the horizontal plumbing in the basement (leaving the vertical pipes in the walls), re-plumb with PEX and install a new high-efficiency boiler. I need a thread - male or female at the end of each pipe to attach the new PEX fitting. For the life of me, I can not budge the old fittings. I've tried heating them and using a big 'ol pipe wrench, no luck; I'm afraid if I put too much torque on those old pipes I'll damage something up in the walls. What do I do? I've already started cutting out the main pipes, but have made sure there is a fitting at the end of each supply and return. Help!!