The problem is, there are 2 thickness: up thickness and proof thickness. See the drawing below. Imagine air gap for up and down surfaces like that we sure there is no torque from these surfaces and we sure this cost nothing in energy if we turn in circular threads. But external and internal have a torque, for me, sure ;)
Drawing: look at "Z" axe
It's not a problem to cancel the forces on these surfaces if we put small air gap (with gaskets) and put in a "sandwich" in 2 fixed circular threads. This cost nothing (in theory) about energy.
I add a drawing with a square thread in a "sandwich" of circular threads. Air gap at up and down surfaces.