How to Read and Understand Bolt Grades

Have you ever looked at the head of a bolt, saw some markings or numbers on it, and then scratched your head and thought, "I wonder what those mean?" This guide will help you understand how to read bolt grades so you can choose the right bolt for your application.

Bolt head markings allow you to visually identify bolt grades quickly and easily. But first, you should know the difference between standard and metric fasteners. The ratings for standard bolts, often identified as SAE, and metric hardware aren't interchangeable. Metric and SAE bolts are each specific in the way they are rated and read. Whether metric or standard, all bolts are sized by diameter, thread pitch, and length—in that order. So, for example, an SAE 1/2-16 x 2 bolt is ½" in diameter, has 16 threads per inch, and is 2" long. An M12-1.5 x 50 metric bolt is 12mm in diameter, has a thread pitch of 1.5mm, and is 50mm long.

As mentioned above, bolts are available in SAE and metric, and each is graded differently. Here's a look at the more common bolts and their grades, along with their markings and what they mean.

Grade 2:

This is a bolt head with no markings and is made of low or medium carbon steel. These are the most common household grade of bolts and also the least expensive. Typically, this is the grade found in fasteners such as machine screws and carriage bolts.

Grade 5:

Identified by 3 radial lines, these are made of medium carbon steel. They're commonly used in automotive applications for their strength since they've been case hardened. Case hardening means the bolts are heated enough during production that the outside of the bolt is harder and stronger than the inside of the bolt.

Grade 8:

These bolts are identified by 6 radial lines and made from medium carbon alloy steel. Fully heat-treated, these bolts are used in heavy manufacturing. While the heat treating of the bolt makes them extremely hard throughout, they can also be somewhat brittle, and will snap off rather than bend when under pressure from extreme loads. 

18-8 Stainless Steel:

Markings on stainless steel bolts are not universal and can vary from head to head. They're typically made from steel alloy with 17-19% chromium and 8-13% nickel. Stainless steel won't tarnish or rust, offering a clean, consistent look. Durable and strong, they're also well suited for use in harsh corrosive environments and will withstand extreme temperature fluctuations.

Class 8.8:

Made from medium carbon steel, class 8.8 bolts are commonly used in processing equipment, engines, and motors. While highly reliable, class 8.8 bolts do not perform well in environments where temperatures can vary greatly.

Class 10.9:

Made from medium carbon steel, medium carbon steel alloy, and low carbon boron steel, these bolts are ideal for fastening large, forged parts as well as for use in auto assembly. They offer excellent wear resistance.

Class 12.9:

Made from alloy steel, this is the highest metric class for strength. A strong and durable fastener, it exceeds grade 8 bolt characteristics.

A-2 Stainless Steel:

These are similar to SAE stainless steel bolts. Again, the markings on stainless steel bolts are not universal and can vary from head to head. They're made from steel alloy with 17-19% chromium and 8-13% nickel, and won't tarnish or rust for a clean, consistent look. Durable and strong, stainless steel bolts are well suited for harsh corrosive environments and can withstand extreme temperature fluctuations.

In addition to the bolt head markings, there are other critical mechanical properties of bolts to understand when selecting the correct fastener. Proof load measures the amount of weight the bolt itself can support. Minimum yield strength is the load at which a bolt shows a specific permanent deformation such as stretching, cracking, or bending. Tensile strength is the maximum load in tension, or pulling apart, that a bolt can withstand before actually breaking or fracturing. 

A bolt really is a simple fastener, and now the markings on it should be just as simple to understand with the help of this guide. If you have bolts that need fastening, let Zoro help you with our broad range of fastening tools, from ratcheting wrenches to torque multipliers.

The product statements contained in this guide are intended for general informational purposes only. Such product statements do not constitute a product recommendation or representation as to the appropriateness, accuracy, completeness, correctness or currentness of the information provided. Information provided in this guide does not replace the use by you of any manufacturer instructions, technical product manual, or other professional resource or adviser available to you. Always read, understand and follow all manufacturer instructions.