A dissolvent oil that I've used with success for stubborn stuck metal parts, is KANO Aerokroil; it magically creeps to where it needs to. It just requires patience with repeated applications of oil soakings and wrenching with a swatch of leather between the knob and teeth of a vice grip pliers. Then once you get it loose, clean the gunk with Mean Green Cleaner & Degreaser. Then if you have it loose and it still won't come off, then use a flat-tipped metal punch and rubber hammer to coax it to slide off from behind.
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Last edited by lmc8; 06-25-2016 at 08:18 AM.