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Reading Bolt Heads

Reading Bolt Heads


“Do you think I’m a fool? That the Commander doesn’t know every bolt- every weld- in his ship?”

Galaxy Quest (1999)

Did you know you can get a general idea of how strong a bolt it just by looking at the head? It may not apply to every bolt, but it does apply to most. Depending on whether you go by American or metric standards, the process can range from arbitrarily complicated (American) to actually pretty straight forward (metric).


The Society of Automotive Engineers created a standard called SAE J429 Mechanical And Material Requirements for Mechanical Fasteners. This applies certain rules and regulations to the production of inch bolts, screws, and other small fasteners. The SAE standards are not particularly easy to follow without actually knowing what each category offers.

Inch bolts manufactured following SAE standards are assigned a Grade between 2 and 8, with two having the lowest tensile strength and eight having the highest. You can (usually) tell what grade a bolt is just by looking at the head. Each grade is designated by a number of radial lines. Over-complicating things, however, is the fact that the number of lines doesn’t correlate directly to the grade. You will have 2 less radial lines on the head than the grade of the bolt. For example, a Grade 5 bolt will have 3 lines, a Grade 8 bolt will have 6.

Grade 2 bolts will have no lines, which can become confusing because many bolts may just not have lines. As such, you may not always be sure if you have an SAE Grade 2 bolt or just an unmarked bolt.

ISO 898-1

For metric bolts, screws, and studs, the standard system is called ISO 898-1. While SAE is considered the standard for inch measurements, there are other systems as well. For metric there a few others, but ISO 898 is by and far the standard.

ISO 898 designations are also far easier to read and identify. Rather than using an arbitrary Grade system, the bolt’s strength values are printed right on the head with just 2 easy to understand digits, like 8.8 or 10.9. The first digit represents the tensile strength (maximum tension load) of the bolt in 1/100th megapascals. The second is the bolt’s yield strength (maximum load before deforming), shown as 1/10th of the percentage of tensile strength.

Using 8.8 as an example, the tensile strength of the bolt would be 800 MPa, and the yield strength would be 640 MPa (80% of 800). For 10.9, your values would be 1000 MPa tensile and 900 MPa yield. These numbers are rough estimates and vary slightly depending on the size of the bolt.

You will also often find a manufacturer’s marking on the bolt head regardless of which system it uses. These markings can be anything from initials to small icons or logos. There is generally no standard for manufacturer’s markings, they fit them wherever they can without interfering with the important information.

That covers the two most common standard systems, but there are plenty of others. If there’s one thing that’s certain in this world, it’s that standards are relative. At least now we can make sense of some of it.