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.

Thread Class 牙等级