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See also: thread classes for bolts sized in metric units.
Classes of fit are tolerance standards; they set a plus or minus figure that is applied to the pitch diameter of bolts or nuts. Thus they describe how much bigger or smaller a thread can be than the ideal size specified by the thread series standard. The classes of fit used with almost all bolts sized in inches are specified by the ANSI/ASME Unified Screw Thread standards (which differ from the previous American National standards).
There are 3 basic classes, numbered from loosest fit to tightest. Classes for external (male) threads have an “A” suffix, for example, “2A”, and classes for internal threads have a “B” suffix. (A special class, 5, is discussed later.) The actual tolerances are calculated for each size, based on the pitch, major diameter and length of engagement.
The loosest fit. Only found on bolts ¼ inch in diameter and larger. Most people will never encounter a fastener with this class of fit. It is mainly used in military hardware where the ability to quickly assemble damaged threads under dirty conditions has the highest priority.
If a class 1B nut with the smallest allowable diameter were fitted to a class 1A bolt with the largest allowable diameter, there would still be a space between them. This is called the “allowance”. Class 1A threads are not coated or plated.
2A/2B is by far the most common class of fit. Like class 1A, class 2A bolts have an allowance. They may be coated.
The closest fit. No allowance. In fact, if the nut's minor diameter was at the minimum permitted, and the bolt's major diameter at the maximum permitted, they would be everywhere in contact, though this situation is highly unlikely.
Class 3A is used for high-strength bolts.
The tolerances for the internal threads are 30% greater than the tolerances for the external thread of the matching class, but there is no allowance.
No allowance; that is only applicable to the 1A male thread. One reason for this is that it would be wasted, since it is very difficult to plate or coat the inside of a nut.
No allowance; that is only applicable to the 2A male thread.
The tightest fit; used for high-strength bolts.
Class 5 is an “interference fit”, that is, the external and internal threads overlap, so when they are assembled one or both must be subtly deformed.
ANSI/ASME B1.1-1989.
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Last revised: 3 October 2006.