ASME B4.2-1978 (R2020)

This standard describes the ISO system of limits and fits for mating parts as it is approved for general engineering usage in the United States of America. It establishes:

(1) the designation symbols used to define specific dimensional limits on drawings, material stock, related tools, gages, etc.,

(2) the preferred basic sizes (first and second choices),

(3) the preferred tolerance zones (first, second and third choices),

(4) the preferred limits and fits for sizes (first choice only) up to and including 500 millimeters, and

(5) definitions of related terms.

Tolerance zones for basic sizes in the range from 500 to 3150 mm are specified in Appendix B. The general terms “hole” and “shaft” can also be taken as referring to the space containing or contained by two parallel faces of any part, such as the width of a slot, the thickness of a key, etc.