Although not strictly necessary, it is recommended to debark the tree butt and remove the sapwood, the outer wood with the latest growth. Despite watering, these outer layers between the heartwood and bark can contain nutrients, making the pump susceptible to weathering, fungi and wood-boring insects.
Removing this outer layer is done by squaring the trunk. This can be done both before and after the boring process. A practical benefit of squaring beforehand is that you take excess weight off the trunk which makes it lighter to handle.
The trunk is fixed with heavy iron clamps low above ground level. After removing the bark with the barking shovel, we opted in this experiment to square the trunk in an octagonal shape using broad axes. You can use a carpenter's adze, but this works a lot slower. The octogonal shape works best for removing the sapwood and fixing the trunk on the boring trestle and in the well. After squaring, the diameter of our pump body was reduced to about 25 cm. However, pump bodies are found in all kinds of forms, square ones usually measure about 30 by 30 cm. We know from testimonials and images that the squared pump body was sometimes smoothed with a block plane.
The liquid manure pump in the MOT collection proves that not every pump was squared. The above-ground part of this pump had only been debarked and left in a rough shape, only one meter at the bottom was squared, presumably to make it easier to fix in the cesspool.
If you want to learn how to square tree trunks yourself, you can join the Balkenhak-Doe, one of the workshops of the MOT.