Systems of Bolt Grades

Bolts are made of different grades of steel. The bolt grade determines the maximum amount of stress the bolt can handle. Tensile strength is the amount of pull the bolt can withstand before breaking. The method used for tightening the bolt is dependent upon the bolt grade. The grades are indicated on top of the bolts. They show up as raised numbers or dashes. There are three main systems of bolt classification: SAE, Metric and ASTM.

• SAE Bolt Grades: The standards for bolt strength grades in the U.S. They are set according to a system developed by the SAE. SAE stands for Society of Automotive Engineers. The SAE marking system uses raised dashes on the bolt head. The dashes are used to indicate strength.

• Metric Bolt Grades: Set according to rules set by the ISO. ISO stand for International Standards Organization. The grades consist of two numbers separated by a dot. The property class is expressed in raised or depressed numerals. The numerals are on top or on the side of the bolt head.

• ASTM Bolt Grades: Set by the American Society for Testing and Materials. Its strength grades are indicated by the letter A. There are also three numerals stamped on the bolt head.

Bolt Grade Numbers

The most common grading system used for bolts in the U.S. is the SAE system, but you can find metric grading systems as well. There are several grades of bolts but the three common grades for SAE are 2, 5 and 8. The common class (grades) for metric are 5.8, 8.8, 10.9 and 12.9. Each grade has a specific bolt strength. No matter the system used, the higher number means the stronger the bolt. For most home and automotive applications, you won't need anything more than a grade 8 bolt strength or metric class 10.9.

Important metrics to understand:

• Yield Strength: The maximum load at which a material exhibits a specific permanent deformation.
• Proof Load: An axial tensile load which the product must withstand without evidence of any permanent set.