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Selection of tolerances and fits 1) The hole-base system should be preferred. In industrial production, it is much more difficult to machine holes compared to shafts that require the same level of precision. Moreover, hole machining and inspection typically involve the use of fixed tools and measuring devices such as drills, reamers, broaches, and gauges. If the tolerance range for holes is fixed, it becomes possible to reduce the number of such tools and measuring devices. Therefore, in general, the hole-base system is always preferred in order to reduce the variety and quantity of tools and measuring devices, thereby lowering production costs. However, in some cases, the base axis system must be used; for example, when the shaft component that fits into a hole is a standard part, such as in the case of the fit between the outer ring of a rolling bearing and the bearing hole, the base axis system should be employed. Furthermore, when shaft sections of the same size need to be combined with multiple holes that have different fit requirements in order to achieve various fit properties, the base shaft system is preferable. In the case of the piston pin shown in Figure 11, its fit with the piston hole is a transitional fit, while its fit with the connecting rod bushing is a clearance fit. If the base hole system were to be used, the piston pin would have to be manufactured as a stepped shaft that is larger at both ends and smaller in the middle, which obviously makes processing and assembly more difficult. (2) Commonly used and preferentially selected tolerance zones and fits l) Commonly used and preferred tolerance zones **The standard specifies 20 different tolerance grades as well as various basic deviations for holes and shafts ； Furthermore, any basic deviation can be combined with any standard tolerance to create a large number of tolerance zones of different sizes and positions, thereby resulting in a wide range of clearances, interferences, or transitional fits. In practical applications, using too many tolerance zones is not conducive to production. In order to reduce the variety and quantity of fixed tools, measuring instruments, as well as the types and specifications of processing equipment, and thereby lower production costs, it is necessary to impose limits on the number of tolerance zones. Therefore, **the standards define general tolerance zones, commonly used tolerance zones, and preferred tolerance zones.