A six-sided internally threaded fastener which is both thicker and wider across the flats than a same-sized finished hex nut. Nuts in sizes 7/16 & smaller shall be double chamfered. Larger sizes are either double chamfered or chamfered on top with a washer faced bearing surface.
This is the strongest of all comparably-graded nuts because of its greater length of thread engagement and greater resistance to dilation (widening or stretching). Grade-A nuts are used with low-carbon heavy hex bolts. Grade-C nuts are recommended for use with A-325 structural bolts. Grade-2H nuts are recommended for use with bolts in high-pressure and high-temperature service. Grade-DH nuts are recommended for use with A-490, Type-1 structural bolts and Grade-DH3 nuts for use with A-490, Type-3 structural bolts.