I don't think the threads will corrode on these plugs, as the sealing washer is between the head of the bolt and the casing so there is oil in the threads. Certainly no sign of corrosion on these that had not been touched for 10 years. I did start the impact gun on the lowest setting then increased it until the bolt started to move. if it didn't move the next stage was to heat the gearbox then fit a piece of short pipe over the plug and use some plumbers pipe freeze spray to freeze the plug.
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