Grades of Bolts

What are Bolts?

Bolts are steel fasteners with heads and external male threads, used to assemble two unthreaded objects, typically with a nut. Using the correct grade ensures a solid, durable join. Here’s a rundown of bolt grades in U.S. customary and metric sizes.

Bolt Grade Systems

Bolts come in different steel grades, which dictate the maximum stress they can handle. Tensile strength is the amount of pull a bolt can endure before breaking. Tightening methods depend on bolt grade, indicated by raised numbers or dashes on the bolt head. There are three main classification systems: SAE, Metric, and ASTM.

SAE Bolt Grades

The SAE (Society of Automotive Engineers) sets U.S. standards for bolt strength. The SAE marking system uses raised dashes on the bolt head to indicate strength.

Metric Bolt Grades

These are set by the ISO (International Standards Organization). The grades consist of two numbers separated by a dot, with raised or depressed numerals on the bolt head.

ASTM Bolt Grades

The ASTM (American Society for Testing and Materials) grades bolts using the letter A, along with three numerals stamped on the bolt head.

Bolt Grade Numbers

The SAE system is the most common in the U.S., but metric grading systems are also prevalent. Common SAE grades are 2, 5, and 8. Metric grades include 5.8, 8.8, 10.9, and 12.9. Higher numbers indicate stronger bolts. For most home and automotive uses, SAE grade 8 or metric class 10.9 bolts are sufficient.

Important Metrics to Understand:

  • Yield Strength: Maximum load at which a material shows permanent deformation.
  • Proof Load: An axial tensile load that a product must withstand without permanent deformation.

SAE Bolt Grades:

  1. Grade 2:
  • Head Marking: None
  • Material: Low/medium carbon steel
  • Proof Load: 55,000 psi (1/4″ to 3/4″), 33,000 psi (7/8″ to 1 1/2″)
  • Yield Strength: 57,000 psi (1/4″ to 3/4″), 36,000 psi (7/8″ to 1 1/2″)
  • Tensile Strength: 74,000 psi (1/4″ to 3/4″), 60,000 psi (7/8″ to 1 1/2″)
  1. Grade 5:
  • Head Marking: Three radial lines
  • Material: Medium carbon steel, quenched and tempered
  • Proof Load: 85,000 psi (1/4″ to 1″), 74,000 psi (over 1″ to 1 1/2″)
  • Yield Strength: 92,000 psi (1/4″ to 1″), 81,000 psi (over 1″ to 1 1/2″)
  • Tensile Strength: 120,000 psi (1/4″ to 1″), 105,000 psi (over 1″ to 1 1/2″)
  • Grade 8:
  • Head Marking: Six radial lines
  • Material: Medium carbon alloy steel, quenched and tempered
  • Proof Load: 120,000 psi
  • Yield Strength: 130,000 psi
  • Tensile Strength: 150,000 psi
  1. Grade A325 (ASTM Specification):
  • Head Marking: A325
  • Material: Carbon or alloy steel, with or without boron
  • Proof Load: 85,000 psi
  • Yield Strength: 92,000 psi
  • Tensile Strength: 120,000 psi
  • Recommended for structural steel connections in heavy construction, similar to SAE Grade 5 bolts.

Metric Bolt Grades:

  1. Class 8.8:
  • Head Marking: 8.8
  • Material: Medium carbon steel, quenched and tempered
  • Proof Load: 580 MPa (<16 mm), 600 MPa (16-72 mm)
  • Yield Strength: 640 MPa (<16 mm), 660 MPa (16-72 mm)
  • Tensile Strength: 800 MPa (<16 mm), 830 MPa (16-72 mm)
  1. Class 10.9:
  • Head Marking: 10.9
  • Material: Alloy steel, quenched and tempered
  • Proof Load: 830 MPa
  • Yield Strength: 940 MPa
  • Tensile Strength: 1040 MPa
  1. Class 12.9:
  • Head Marking: 12.9
  • Material: Alloy steel, quenched and tempered
  • Proof Load: 970 MPa
  • Yield Strength: 1100 MPa
  • Tensile Strength: 1220 MPa

No matter what type of bolts you need, The Gmetalparts has you covered. We stock bolts, washers, and nuts for a secure fit. Looking for a specific bolt? The Gmetalparts delivers online orders to your location.

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