That's a neat idea but, sadly, I don't think it will work. The reason I say that is that directly driving that threaded rod will not give sufficient power to move the piston against water pressure when trying to surface (especially from any depth) as well as air pressure when trying to submerge. And this is compounded by the 3 1/2" diameter of the piston. That would probably work for, say, a 1" diameter trim tank using a fine thread but even there it would be much more efficient geared down.

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