Heavy hex nuts are specified for use with bolts or studs having any of the following characteristics: higher strength, zinc coatings or nominal thread size larger than 1-1/2″.

Heavy hex nuts, as defined in ANSI B18.2.2 are greater in width and thickness than standard hex nuts. ASTM A563 is the most commonly referenced ASTM specification for carbon and alloy steel nuts. It includes 8 grades and covers sizes from 1/4″ to 4″.

ASTM A194 covers carbon and alloy steel nuts that are intended for use in high pressure and/or high temperature applications. It also covers sizes from 1/4″ to 4″ and includes dozens of grades. SAE J995 is another frequently referenced specification for carbon and alloy steel nuts. It covers sizes from 1/4″ to 1-1/2″ and includes three grades.

The geometry, dimensions and tolerances for Unified Inch screw threads are defined in ANSI B1.1. This specification includes information for UNC, UNF, UNEF and constant pitch (4UN, 6UN, 8UN etc.) threads.

Selecting the appropriate nut for an application is straightforward if the mating bolt or stud was produced to an ASTM standard. The heavy hex nut selection table is a summary of the ASTM recommendations.

Heavy Hex Nuts

Heavy hex nuts are very similar to finished hex nuts. The main two differences are going to be the width (Across the flats) of the nut and the available grades. While they are available in many different material specifications, they are not available in the common grades 2, 5, and 8 that you will get used to in standard finished hex nuts. The most common heavy hex nuts are the 2H (Heavy and Hardened) and the A563 (lower right) which is often used as a heavy version of the low carbon grade 2 nut.

Example:

1/2” nut X 1.75 = 7/8″ OD

1” nut x 1.75 = 1 3/4″ OD