[fintstone]

If it is the threads that you are thinking are boogered up that are causing your problem, I would take a fine file or pick and clean them up enough/fix them enough to tighten it down more. Those back threads will prevent you from screwing it on far enough if bad (raised), but their absence probably will not. The harder metal in the line will help clean up the threads some once you clean them up. I would try very hard to save that thermostat housing...both for cost and because you may bugger up your front lines removing it. They may have never been off. Those are the ones that usually cause folks problems.