How to Determine Hex Bolt Grade for Automobile Hexagon Bolts
Hex bolt grade refers to the grade of steel used to make screws. This determines the amount of stress that can be applied to the fastener, called "tensile strength."

The grade of the hex head screw can be identified by the marking on the screw head. But first, why are there standard markings on the head to indicate rank? Well, Congress passed a law in 1990 that set standards for fasteners sold in the United States. Known as Public Law 101-592, its purpose is to provide the public with a standard for easy identification of fastener strength, hardness, grade, and composition.

Fasteners generally fall into three categories - SAE, Metric, and ASTM

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