What is the difference between a standard hex nut and a heavy hex nut?

Hex NutA standard hex nut, also referred to as a finished hex nut, has a smaller width across the flats and corners compared to a heavy hex nut, and a heavy hex nut is slightly thicker than a standard hex nut of the same nominal size. In fact, heavy hex nuts are exactly a 1/8” larger across the flats than a standard hex nut in all sizes, requiring a 1/8” larger socket or wrench to install. Also, heavy hex nuts have a higher proof load strength compared to a hex nut according to ASTM A563.

A563 Mechanical Properties

Grade Style Size, in. Proof Load, ksi Hardness, HBN
Plain Galvanized
A Hex 1/4 - 1-1/2 90 68 116 - 302
Heavy Hex 1/4 - 4 100 75 116 - 302
B Heavy Hex 1/4 - 1 133 100 121 - 302
Heavy Hex 1-1/8 - 1-1/2 116 87 121 - 302
C / C3 Heavy Hex 1/4 - 4 144 144 143 - 352
D Heavy Hex 1/4 - 4 150 150 248 - 352
DH / DH3 Heavy Hex 1/4 - 4 175 150 248 - 352
For UNC, 8UN, 6UN, and Coarse Pitch Threads

By default, the ASTM A194 nut specification requires the dimensions to conform to the American National Standard Heavy Hex Series unless noted otherwise.

Alternatively, the ASTM F594 nut specification calls for the dimensions to be in accordance with ASME B18.2.2 for Hex Nuts.

Lastly, ASTM A563 requires the style of nut (for example, heavy hex) to be listed on the inquiry.  However, All A563 nuts will be heavy hex by default except for Grade A in sizes 1/4”-1-1/2” diameter.

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