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The flat area at the top of the horizontal threaded hole in the body is there to avoid the need for support material to be used for the print. If you move up the upper half of the inner surface of such a hole the surface starts to form an overhang, and the angle of the overhang increases as you move up towards the top of the hole. At some point that overhang will get to the point where the outer ends of new layers of extruded filament will be being laid down effectively over thin air, and they will sag down and distort the shape of the hole as a result of being unsupported.

One way of dealing with that is to have the slicer generate support material that would support the steepest overhanging parts of the upper surface of the hole, but I wanted to avoid that as cleaning such support out of a threaded hole would be a pain, and unlikely to leave a good enough finish on the inside of the threaded hole (good enough in terms of allowing the plunger to move/turn smoothly ).

My solution was to stop the curved surface of the inside of the hole at the point where the overhang would be expected to get too extreme and at that point have the wall go straight up to what would be its maximum height, and then finish the hole by bridging horizontally to the opposite side of the hole ('bridging' not suffering from the fact that it is printing over thing air because both ends of the extruded filament strands are held in place by the underlying print).