Usually on a big metal part like that a propane torch is not enough. They sell MPS cylinders that have the same physical shape and use the same nozzle as a propane torch, and give a lot more heat.

I would say try with the MPS if you can find one or bring the piece to a local garage and ask them to heat it with their acetylene torch, then use the easy out.

Another trick, if there is enough bolt stud out of the knuckle, you can have a nut welded to the bolt and unscrew it while it is still hot.

If that doesn't work, the last option would be to drill and tap new treads.

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