What are the marks shown on the head of a bolt?
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 organisation that accepts the responsibility that the fastener meets specified requirements. The grade mark is a standardised 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. The bolt head shown at the side indicates that it is of property class 8.8 and ML is the manufacturer’s mark. 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 added (possibly with illegitimate marks).
