Bolts are headed steel fasteners with external male threads. They are used to assemble two unthreaded objects, typically with the use of a nut. Purchasing the right grade of bolt will make sure you have a solid, durable joining of two materials. This guide will show you the different grades of bolts in both U.S. customary sizes and metric.
Systems of Bolt Grades
Bolts are made of different grades of steel. The grade of a bolt determines the maximum amount of stress (tensile strength) that 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 also dependent upon the grade of the bolt. The grades are indicated on top of the bolts 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. are set according to a system developed by the SAE (Society of Automotive Engineers). The SAE marking system uses raised dashes on the bolt head to indicate strength.
- Metric Bolt Grades—Metric grades are set according to rules set by the ISO (International Standards Organization). The grades consist of two numbers separated by a dot. The property class is expressed in raised or depressed numerals on top or on the side of the bolt head.
- ASTM Bolt Grades—The ASTM grade standard is set by the American Society for Testing and Materials. Its strength grades are indicated by the letter A plus three numerals stamped on the bolt head.
Bolt Grade Markings
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 to grade the bolts, 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.
SAE Bolt Grades
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 that’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
Metric Bolt Grades
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
No matter the types of bolts you need for your home or mechanical project, The Home Depot has the bolts, washers and nuts you need for a secure fit. The Home Depot makes it simple to find the hardware, tools and anything you need to get the job done right. Shop online today and start that DIY project today.
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