Grades of Bolts

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 stands 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.

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.

SAE Grade 2 Bolts:

• Head Marking: No marking
• Material: Low or medium carbon steel
• Size Range: 1/4-inch to 3/4-inch
• Proof Load (psi): 55,000
• Min. Yield Strength (psi): 57,000
• Min. Tensile Strength (psi): 74,000
• Size Range: 7/8-inch to 1 1/2-inch
• Proof Load (psi): 33,000
• Min. Yield Strength (psi): 36,000
• Min. Tensile Strength (psi):60,000

SAE Grade 5 Bolts:

• Head Marking: Three radial lines
• Material: Medium carbon steel, quenched and tempered
• Size Range: 1/4-inch to 1-inch
• Proof Load (psi): 85,000
• Min. Yield Strength (psi): 92,000
• Min. Tensile Strength (psi): 120,000
• Size Range: Over 1-inch to 1 1/2-inch
• Proof Load (psi): 74,000
• Min. Yield Strength (psi): 81,000
• Min. Tensile Strength (psi): 105,000

SAE Grade 8 Bolts:

• Head Marking: 6 radial lines
• Material: Medium carbon alloy steel, quenched and tempered
• Size Range: 1/4-inch to 1-1/2-inch
• Proof Load (psi): 120,000
• Min. Yield Strength (psi): 130,000
• Min. Tensile Strength (psi): 150,000

Grade A325:

This bolt is an ASTM specification bolt. It's readily available in many home improvement and automotive stores. It’s virtually identical in terms of chemical and physical strengths and properties as an SAE Grade 5 bolt.

• Head Marking: A325
• Material: Carbon or Alloy Steel with or without Boron
• Size Range: 1/2-inch to 1 1/2-inch
• Proof Load (psi): 85,000
• Min. Yield Strength (psi): 92,000
• Min. Tensile Strength (psi): 120,000

Engineers usually recommend ASTM A325 bolts for structural steel connections in heavy construction projects. On the other hand, SAE grade 5 bolts are common in OEM-type applications. OEM stands for Original Equipment Manufacturer.

Class 8.8:

• Head Marking: 8.8
• Material: Medium carbon steel, quenched and tempered
• Size Range: All sizes below 16-mm
• Proof Load (MPa): 580
• Min. Yield Strength (MPa): 640
• Min. Tensile Strength (MPa): 800
• Size Range: 16-mm to 72-mm
• Proof Load (MPa): 600
• Min. Yield Strength (MPa): 660
• Min. Tensile Strength (MPa): 830

Class 10.9:

• Head Marking: 10.9
• Material: Alloy steel, quenched and tempered
• Size Range: 5-mm to 100-mm
• Proof Load (MPa): 830
• Min. Yield Strength (MPa): 940
• Min. Tensile Strength (MPa): 1040

Class 12.9:

• Head Marking: 12.9
• Size Range: 1.6-mm to 100-mm
• Proof Load (MPa): 970
• Min. Yield Strength (MPa): 1,100
• Min. Tensile Strength (MPa): 1,220