Jam Nuts are often used to lock or "jam" something in place without clamping against another object; Jam Nuts are typically low profile compared to a standard nut. Grade A per ASTM A563 hardness: HRB 68 Minimum. Thread Dimensions ANSI B1.1 UNC and UNF Class 2B. Zinc plating, retards the corrosion rate in a normal atmosphere.