The ISO fits of shafts and hubs assume a hole that can be made to a few hundredths of a millimetre. A printed hole cannot, so the designer accepts the printer’s tolerance and designs around it [1]. A clearance fit needs a gap of at least twice the tolerance. An interference fit in a round hole has to stretch the whole wall, and its force swings with every tenth of a millimetre, so the rules replace the circle: a hexagon or a square that bends instead of stretching, crush ribs that flatten as the part goes in and give the same grip over the whole scatter of hole sizes, and grip fins that flex elastically for a part that is taken out and put back [2]. Where precision is needed anyway, the answer is to make the dimension adjustable (a slot, opposing screws, a shim) or to ream the hole. This chapter holds the rules and the numbers; the mechanics of the press fit, the contact pressure and the torque it carries stay in Fits in printed parts of the shaft-hub chapter, and the bearing seat in Bearings in printed parts. Our test pushes pins out of round, hexagonal and ribbed holes on the UTM, at assembly, after a month, and after reassembly.