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...I continue to have very good luck with difficult to remove threaded stuff using a 50/50 mix of ATF (from the auto store) and acetone (from the paint dept at Home Depot) as the penetrating oil. You need to keep it in a bottle made from a plastic that is acetone proof..I use an old empty small TriFlow bottle. Wipe it off any rubber or plastic parts you get it on.

It's good to give it 5 or 10 minutes to soak in, then reapply a little as you start to work.

I just took apart a very old Zeus bike, and there were a number of things (fixed cup, cranks) that would not have come off intact without this stuff.