Inch Thread Accuracy Grade

A thread is a shape having a uniform spiral protrusion on a section of a solid surface or an inner surface. The inch thread has external and internal threads. Two different threads also differ in accuracy level. For uniform inch thread, the external thread has three thread grades: 1A, 2A, 3A; the three grades of the external thread are: 1B, 2B, 3B, all of which are clearance fits. The higher the level number, the tighter the fit.  inch thread

In the inch thread, the deviation only specifies the 1A and 2A levels, the 3A level deviation is zero, and the 1A and 2A level deviations are equal. The larger the number of grades, the smaller the tolerance.

★ Class 1A and Class 1B, very loose tolerance class, suitable for tolerance fit of internal and external threads.

★ Grades 2A and 2B are the most common thread tolerance grades specified by the British series of mechanical fasteners.

★ 3A and 3B grades, screwed together to form the tightest fit for tight tolerance fasteners for critical safety design.

★ For external threads, 1A and 2A have a tolerance, and 3A does not. The 1A tolerance is 50% greater than the 2A tolerance and 75% greater than the 3A. For internal threads, the 2B tolerance is 30% greater than the 2A tolerance. Level 1B is 50% larger than 2B and 75% larger than 3B.

The table below is for tolerance requirements for inch thread.

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