The most common designation used for standard industrial bolts and nuts is Coarse Thread Series (UNC/UNRC). Because coarse thread fasteners are less likely to cross thread, they can be beneficial in applications where quick assembly is required.
When looking at more precise applications, Fine Thread Series (UNF/UNRF) fasteners are more commonly used. Fine thread fasteners have higher tension strength due to the larger tensile stress areas. However, fine thread applications require a longer engagement than coarse thread applications to ensure stripping does not occur.
The specified thread forming method for several ASTM standards is 8 – Thread Series (8UN) . This includes A193 B7, A193 B8/B8M, and A320, and is only used for one inch diameters and larger.
Coarse Thread Series
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8 - Thread Series
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Metric Series
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Fine Thread Series
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