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Soak the remaining screw with PB Blaster. Take a pin punch and place it on top of the screw and hit the punch with a few good whacks. The vibration will help break the oxidation bond of the dissimilar metals. The PB will then work its way in. Do not use a wrench or ratchet to loosen the screw. Instead use a hand impact driver with a 3/8” drive 6 point socket (not a 12 point) continue striking the hand impact while twisting and keeping the screw wet with PB. This should loosen the screw. If and when it starts to loosen, keep turning it in the same direction only. Do not try and work it back and forth as this will only fatigue the screw more. If it breaks, you will have to drill it and use a ez out. It may however be just easier to drill new holes next to the old holes and tap rather than extract the original screws. Replace the old screws and use never seize.
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You could also use one of these. 1/4” impact driver.
If you use the electric impact go very easy with it as it can wring the head off in a instant. Again do not reverse direction.
Personally, I believe the hand impact driver would be best in this situation.
Good Luck...🍀
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