• James Abbott - MD Challenge Engineering

Nuts, Bolts & Fasteners - Grades and Classes - What's the Difference?

Making bolts from high tensile steel, grade 4140 at Challenge Engineering in Sydney.

Fasteners, including nuts and bolts, are manufactured to comply with one of two different sizing systems.

The first system is referred to as grades of fasteners, which uses imperial measurements, i.e. inches. They are primarily used in North America where the imperial system is still in common use.

The second system is called classes of fasteners, which uses metric dimensions, i.e. millimetres (mm).

This article will explain some of the differences, how to identify which fasteners are which, and how much torque should be applied.

A brief history of fasteners

The industrial revolution brought about threaded nuts and bolts as we know them.

In 1760, a British company started the first mass production factory of threaded screw bolts. Other factories followed, but there was a problem. The various companies produced different sized threads, nuts and bolts, making life difficult for manufacturers of machinery.

In 1841, Joseph Whitworth found a solution by standardising the sizes across Britain. The thread was standardised at 55 degrees with a set number of threads per inch depending on the bolt diameter.

The Americans originally adopted the same system, but in 1864 William Sellers changed the thread to 60 degrees and proposed differing thread pitches.

The inconsistency within the allied forces caused problems during World War I and II. As a result, in 1948 Britain, the USA and Canada decided to standardise, adopting the Unified thread which was applied in all countries that used imperial measurements.

In more recent years, this was changed to the ISO metric thread which is still in global use today.

Threads had been fully standardised to a metric design, but what about diameters and tensile strength?

Unfortunately, there was no political will to standardise these properties, so there are still two main systems in existence; imperial grades and metric classes.

What do the markings on bolts mean?

Imperial Grades

There are two main grading systems in the US that use imperial diameters. The American Society for Testing and Materials (ASTM) system, and the Society of Automotive Engineers (SAE). They both use numbers to grade their bolts - the higher the number, the higher the tensile strength.

For instance, the SAE system covers diameters of up to 1 ½ inch, and a grade 8 would have a higher tensile strength than a grade 5.

Let’s take a look at the markings you would find on some of the most common SAE bolts. Each of the following would also have letters stamped on them telling you the manufacturer, e.g., A B for Acme Bolts.

This first example below has no markings, which under the SAE system designates Grade 2. This is a low strength bolt with a min tensile strength of 74,000 pounds force per square inch (74 ksi) for diameters of ¾ inch and less. For diameters between ¾ inch and 1 ½ inches, the minimum tensile strength is 60ksi.

Fig 1.Image Credit: wilsongarner.com/bolt-screw-head-markings-and-what-they-mean/

The marking with 3 radial lines designates Grade 5, with tensile strengths between 105-120ksi.

Fig 2.Image Credit: wilsongarner.com/bolt-screw-head-markings-and-what-they-mean/

The final example is a Grade 8 bolt, with a minimum tensile strength of 150ksi covering all diameters.

Fig 3. Image Credit: wilsongarner.com/bolt-screw-head-markings-and-what-they-mean/

ASTM bolts are easier to identify as they use alphanumeric codes, so A325 bolts are stamped with ‘A325’ along with the unique manufacturers marking, as in the example below.

Fig 4. Image Credit: wilsongarner.com/bolt-screw-head-markings-and-what-they-mean/

Metric Classes

The metric nuts and bolts most commonly used in plant and machinery are M5, M6, M8, M10, M12, M16, M20, M24, and M30. These correspond to the diameter in mm, so an M5 bolt has a thread diameter of 5mm.

To find the tensile strength of metric bolts and nuts, there will be a numerical marking on the head, along with a manufacturer’s ID code.

The numbers stamped onto the bolt heads designate the proof load that the fastening can carry per square millimetre before breaking. For instance, the ‘9’ in a 9.8 bolt means it can carry up to 90kg for each square millimetre. The .8 part tells you that at 80% of that load, it will start to stretch or bend.

Therefore a 10.9 bolt takes up to 100kg per sq. mm, and will start failing at 90% of that load.


Imperial fastenings are referred to as grades, and metric fastenings are referred to as classes.

Both have marking systems that make it possible to determine grade and class, which combined with the measurement of the thread diameter allows you to find the tensile strength or proof loading.

To find the relevant tightening torque settings there are various industry charts available for both metric and imperial bolts.

Want to chat to an expert about nuts and bolts, please give me a call (02) 9632 0010.

At Challenge Engineering, in Sydney Australia, we machine custom metal and plastic components, and pride ourselves on great customer service. We can manufacture and deliver anywhere in Australia. Please contact us and find out how we can assist your business.Sign up to Challenge Engineering’s blog, send through a Request A Quote form, email us or phone (02) 9632 0010.

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