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Bolt Grades

Bolt Grades

Bolt grades are a standardized system for identifying the mechanical properties of a bolt, including tensile strength, yield strength, and hardness. These measurements are crucial for engineers and builders to ensure that the bolts they use are suitable for specific applications. Let's delve deeper into bolt grades and their importance.

What are Bolt Grades?

A bolt grade is a classification system used to specify the physical properties of a bolt. These properties include tensile strength, proof load, and yield strength, among many others. Each grade is identified by numbers and/or letters that correlate to the bolt's material and mechanical properties. Bolt grades are essential because they determine the bolt's ability to hold under pressure, its resistance to shearing forces, and its durability.

Classification of Bolt Grades

Bolt grades are broadly classified into metric and imperial (also known as SAE) grades. Each system has its unique identifiers and mechanical properties.

SAE Grades

SAE grades range from 0 to 8, with subgrades like 5.2 or 8.2. The higher the grade, the stronger the bolt. Here is an overview of the most common SAE grades:

  • Grade 2: This is the standard grade for bolts with no markings. They have a tensile strength of up to 74,000 PSI for bolts over ¼ inch.
  • Grade 5: Identified by three radial lines, these bolts have a tensile strength of up to 120,000 PSI.
  • Grade 8: These are high-strength bolts marked with six radial lines and have a tensile strength of up to 150,000 PSI.

Metric Grades

Metric grades range from 3.6 to 12.9. They are marked by their grade number, for instance, "8.8" or "10.9". Higher numbers represent stronger bolts. Here's an overview of the common metric grades:

  • Grade 8.8: Similar to an SAE Grade 5.
  • Grade 10.9: Comparable to an SAE Grade 8.
  • Grade 12.9: The highest metric class, sometimes called "Grade 12".

Importance of Bolt Grades

The grade of a bolt determines its strength, and therefore its function. For instance, a lower grade bolt (Grade 2 or Grade 8.8) would be appropriate for general hardware, whereas a high-grade bolt (Grade 8 or Grade 12.9) would be required for applications that involve high stress or high temperatures.

Choosing the correct bolt grade is crucial for safety, performance, and cost-effectiveness. Using a bolt that is too weak for an application could lead to bolt failure and catastrophic results. Conversely, using a bolt that is stronger than necessary can be unnecessarily expensive.

Understanding Bolt Strengths

To fully comprehend bolt grades, we need to understand the basic mechanical properties that these grades represent.

  • Tensile Strength: This is the maximum load that a bolt can withstand before it fails or breaks. It is expressed in pounds per square inch (PSI) or megapascals (MPa).
  • Yield Strength: This is the maximum load at which a bolt will deform permanently. Beyond this point, the bolt will not return to its original shape after the load is removed.
  • Proof Load: This is the maximum load that a bolt can carry without experiencing any plastic deformation. It's a measure of the bolt's elasticity limit.


Bolt grades are an essential aspect of engineering and construction, ensuring that the materials used are suitable for their intended applications. They help maintain safety standards and improve the longevity and reliability of various structures and machinery. As such, understanding bolt grades and their implications on bolt strength and durability is fundamental for anyone involved in these industries. It is always crucial to ensure the correct bolt grade is used for the right application, to maintain integrity and safety of the structures or systems they are used in.

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