The tolerance of the gage is based on the tolerance of the part it is to inspect. The rule of thumb is to use a maximum of 10% of the part tolerance for the gages. This allows the part manufacturer to use 90% of the tolerance for their manufacturing window, as the gages are made within the part tolerance to assure quality. The 10% is split between the Go and NoGo gages.
For example, if the product has a range of .499 - .500 diameter, 10% would be .0001, which split becomes .00005 (50 millionths). Looking at the tolerance chart you would use class X gages, or better.
The tolerances given in this chart are to be applied to the diameter of the gage. Go cylindrical plugs, and NoGo cylindrical rings are held plus; NoGo cylindrical plugs, and Go cylindrical rings are held minus.
Straightness, taper, and out-of-roundness must be held within 1/2 of the diameter tolerance. Tolerances for Cylindrical Plug Gages used to check the minor diameter of internal threads, and Cylindrical Ring Gages used to check the major diameter of external threads, are specified in the thread standard as Class Z.
Gage Tolerance for Plain Cylindrical Gages:
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