ABSTRACT
Fit is a measure of looseness or tightness between mating threads. Classes of fit are specific combinations of tolerances and allowances as applied to external and internal threads.
For Unified inch series there are three classes for external threads (lA, 2A, and 3A) and three for internal threads (1B, 2B, and 3B). All are clearance fits, which means that these threads assemble without interference. Additionally, there is an interference fit, class 5, providing the tolerance for the external and internal threads which can assure positive interference. The higher the class number the tighter is the fit. The designator 'A' denotes an external thread while 'B' refers to an in ternal thread. Using these definitions class lA/lB represents the loosest fit covered in screw thread standards while class 3A/3B is the tightest of the clearance fits.
