Fastener and Thread Design Menu | ISO Hardware Engineering Data 
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Thread classes are specified to designate the amount of tolerance allowance and installation fit desired. Thread classes are derived from formulas which the pitch diameter tolerances are based on increments of the major (nominal) diameter, the pitch, and the length of engagement of the thread. These formulas and the class identification only apply to Unified threads series.

There are three classes of external (1A, 2A, and 3A) and internal threads (1B, 2B, and 3B). The characteristics and typical application of each class are as follows:

Classes 1A and 1B: These classes are the loosest fit, in that, the largest amount play or clearance in assembly. These are used for work where ease of assembly and disassembly is most important, such as stove bolts and other general rough bolts and nuts.

Classes 2A and 2B: These classes are specified for the typical fit grade of commercial products, such as machine screws and fasteners, where interchangeability and reasonable quality and fit predictability is required. These classes are the most commonly used for commercial and aerospace applications.

Class 2AG: This application is specified to allow for rapid assembly, and to permit the  application of a lubricant or for residual growth due to high-temperature expansion. In these applications, the G is specified to indicate, that when the thread is coated, the thread tolerance and size may not be exceeded by such coating.

Classes 3A and 3B: These classes are specified for high quality commercial,  precision applications and products, such as machine screws, where a close or snug fit is required.

Thread specification examples:

4-40 UNC-2B or .112-40UNC - 2B

1⁄420 UNC-2A or 0.25020 UNC-2A 

1032 UNF-2A  or 0.19032 UNF-2A 

7⁄1620 UNRF-2A or 0.437520 UNRF-2A 

212 UN-1A  or 2.00012 UN-1A 

1⁄420 UNC-3A-LH  or 0.25020 UNC-3A-LH   (LH = Left Hand)

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