Bolt Head Markings Guide — Identify Grade at a Glance
Every bolt head carries markings that reveal its strength grade and material. This guide shows how to read SAE radial-line markings and metric property-class numbers so you can match or replace a fastener with the correct grade and avoid using a weak bolt in a high-stress location.
SAE grade markings (radial lines)
Count the raised radial lines on the bolt head. No lines is Grade 2 (low-strength general purpose), three lines is Grade 5 (standard automotive), and six lines is Grade 8 (high-strength suspension and drivetrain). Some heads also carry a manufacturer mark, which is not a grade indicator.
Metric property classes
Metric bolts stamp a two-number class such as 8.8, 10.9, or 12.9 directly on the head. The first number relates to tensile strength and the second to the yield ratio. Class 8.8 is roughly equal to SAE Grade 5, 10.9 to Grade 8, and 12.9 exceeds Grade 8.
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