Perhaps instead of banging away with the impact wrench set on low torque (didn't work for me after 30 min/stuck plug), massive vibration would bust them loose. Recall some threads about keys stuck in "ignition" lock. Some have sucessfully vibrated keys out with vibrating sander held on the key. Think of some tool or device that would vibrate the begesus out of the stuck plug. Maybe an air scaler or something like that. Just thinking.

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