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There must be a pressure on the up and down forces otherwise water fills the narrow gaps.
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Not like you think, the gasket is only there for prevent water to fill the air gap, you can think with P=0 bar in the air gap. Sure water will put forces on surface of the gasket [because we have X bars in one side due to the water, and this change with altitude, and 0 bar in other side (air gap)] but the surface of the gasket can be so low that we can considered the torque = 0 on the gasket. In this case, I put thr square thread in a sandwich of 2 circular threads, it's very important, the gaskets don't have up and down surfaces, they have only external and internal surfaces but so small...