See also: thread classes for bolts sized in inches
Tolerance class is designated by tolerance grades and tolerance positions.
The tolerance grade is described by a numeral, from 3 to 9 for external threads and 4 to 8 for internal. The higher the number the greater the tolerance. Tables giving the tolerance for a given grade and pitch can be found in references like Machinery's Handbook.
The tolerance position is described by a letter between A and H, though only G and H are used in the United States. Capital letters designate internal threads and lowercase letters external threads. Position is used to describe the fundamental deviation, the clearance between the largest external thread and the smallest internal thread possible at that tolerance grade (what in inch-sized threads is called the allowance). “H” and “h” represent zero fundamental deviation; “G” and “g” a small allowance. The closer the letter is to the beginning of the alphabet, the greater the fundamental deviation.
To designate a tolerance class for an external thread, one gives a tolerance grade with a tolerance position, first for the pitch diameter and then for the major diameter. For example, “4g6g” is the tolerance class for an external thread with grade 4 pitch diameter at position g and grade 6 major diameter at position g. If the two parts of the designation are identical, only one is given, that is, instead of “4h4h” one writes simply “4h”. The tolerance class for an internal thread is similarly constructed, except that instead of the major diameter the minor diameter is described, and the letters are capitalized.
Combining a tolerance class for an external thread with one for an internal thread creates a class of thread fit. A class of thread fit roughly equivalent to 2A/2B is "6g/6H", which means an external thread with tolerance grade of 6 and position g, and an internal thread of tolerance class 6 with position H. “4h6h/4H5H” is a class of thread fit used for some aerospace fasteners, and is roughly equivalent to 3A/3B. The "h" and “H” indicate no fundamental deviation in the fit.
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Last revised: 11 October 2006.