Bolt head markings — what they mean

The marks on a bolt head are the only thing on site that tells you what you are holding. Click any head to read the grade, the standard behind it, the mechanical properties and the nut that belongs with it. Three systems appear on Gulf drawings — metric property classes, inch SAE grades and ASTM specifications — and they are not interchangeable.

Where these figures come from. Metric classes from ISO 898-1 and ISO 3506-1. Inch and ASTM values checked against the published grade tables for SAE J429, ASTM F3125, A307, A193, A354, A449 and F1554, 16-Sep-2026. Where a specification steps down by size, both ranges are shown. Always confirm against the governing edition for a project.

Every head also carries the manufacturer's identification mark. A bolt with a class or grade mark but no maker's mark is not compliant with the standard, whatever it says it is — and is the single most common thing we find on rejected deliveries.

Reading a metric class

A metric property class is two numbers with a dot, and both of them mean something. The first, multiplied by 100, is the nominal tensile strength in MPa. The two multiplied together, times 10, give the yield strength in MPa. So 8.8 is 800 MPa tensile and 8 × 8 × 10 = 640 MPa yield; 10.9 is 1,000 MPa tensile and 900 MPa yield. The class is stamped on the head along with the maker's mark.

One trap: for class 8.8 the standard sets slightly higher figures above M16 than at or below it — 830 MPa tensile instead of 800. The table above shows both.

Matching the nut to the bolt

A nut must be at least as strong as the bolt, or the threads strip before the bolt reaches its load and the joint fails quietly. The pairing is fixed by the standards, not by preference:

A hot-dip galvanized nut must also be tapped oversize — the class mark alone does not tell you it is the right nut for a coated bolt.

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