Usually fastener standards specify two types of marks to be on the head of a bolt. The manufacturer's mark is a symbol identifying the manufacturer (or importer). This is the organization that accepts the responsibility that the fastener meets specified requirements. The "Grade Mark" is a standardized mark that identifies the material properties that the fastener meets. For example 307A on a bolt head indicates that the fastener properties conform to the ASTM A307 Grade A standard.

Both marks are usually located on the top of the bolt head, most standards indicating that the marks can be raised or depressed. Raised marks are usually preferred by manufacturers because these can only be added during the forging process whereas depressed marks can subsequently be added (possibly with illegitimate marks).

The below listed Table is a listing of some of the more common Bolt Head Markings. If you have questions about a bolt head marking or specification, do not hesitate to Contact Us via our electronic form or call us at 877-844-8595.

Hex Bolt Head Markings

Often one will find "extra" marks on a bolt head--marks in addition to those shown above. Usually these marks indicate the bolt's manufacturer.

ASTM A325 Type 2 bolts have been discontinued, but are included above because they can be found in existing structures. Their properties can be important in failure investigations.

While the bolts shown above are among the most common in the U.S., the list is far from exhaustive. In addition to the other bolts covered by the SAE and ASTM standards, there are a host of international standards, of which ISO is perhaps the most well known.

-AFT Team