Classes of Thread
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Classes of Thread
(Tech Tip #5)

Can you explain 'Thread Class'?

Classes of thread fit is one of three factors one must consider when deciding what type of thread is best for a given fastener application. The other 2 are thread form and thread series. We will discuss thread form and thread series in future Tech Tips.

Thread fit is a measure of looseness or tightness of mating threads. For unified inch threads, there are 3 thread classes for external threads- 1A, 2A and 3A - and three for internal threads - 1B, 2B and 3B. All of these are “clearance fits” which indicates that they assemble without interference.

As a general guideline, the higher the class number, the tighter the fit. For example, an assembly which mates class 1A and 1B threads will have a looser fit than an assembly having class 3A and 3B threads.

Classes 1A and 1B are infrequently used, but they do have their applications. They are ideally suited when quick assembly and disassembly are a key consideration in a design. Classes 2A and 2B are the most common thread classes in use, offering a balance of ease of manufacture, cost and performance. Classes 3A and 3B are best suited where close tolerances, safety and strength are required, and thus are frequently specified for socket cap and socket set screws.

It is not necessarily true that tighter thread tolerances and closer fit insure a better quality assembly and performance. Our engineers can assist you in developing the optimal combination of features for your design application. If you would like more information on this or other technical application questions, email us at and we’ll be happy to help you
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