The unit bore is a fitting system. In mechanical engineering, a fit refers to the dimensional relationship of two parts that are to fit together without reworking. Fitting systems are used to manufacture one of the two workpieces that are to produce a fit as uniformly as possible and to relocate necessary tolerances in the other workpiece. Along with the unit shaft, the unit bore is the more widely used of two methods. DIN 7154 defines the unit bore system exactly with the dimensions of the ISO fits. With the unit bore (H) according to the ISO tolerance system, the diameter of the bore is always toleranced with an H-field. Depending on the requirements, the associated shaft diameter will be assigned to a freely selectable tolerance field position. Thus, in most applications a clearance fit is generated, but in principle a transition fit or an interference fit is also possible. The unit bore process enables cost-effective use of the tools employed and has therefore become widespread in mechanical engineering. Common drilling tools have two or more tool cutting edges, which can only be geometrically changed or adjusted with great technical effort. In contrast, it is relatively easy to use the freely positionable bit of a lathe to bring a shaft to its desired dimension if the fit is inadequate.