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.

Metric vs SAE Bolt Head Markings Compared

SAE / Imperial Metric
PLAIN
Grade 2
no lines
 ≈ 4.6
Class 4.6
stamped number
Both are low-carbon and non-structural, but the ranges only overlap — not interchangeable.
Grade 5
3 lines
 ≈ 8.8
Class 8.8
stamped number
The closest true pair: 120,000 psi (827 MPa) against 800 MPa. The everyday under-hood bolt in both systems.
Grade 8
6 lines
 ≈ 10.9
Class 10.9
stamped number
Nearly identical strength: 150,000 psi (1,034 MPa) against 1,040 MPa.
no equivalent
 12.9
Class 12.9
stamped number
No SAE equivalent — 1,220 MPa is above Grade 8. Usually a socket-head cap screw.
Lines mean SAE; a stamped number means metric. The pairings are nearest equivalents for identification, not substitution approvals — match the original grade in any loaded joint.

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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