Fastener Guide

How to Read Bolt Head Markings

  1. Look at the head: raised or stamped lines and numbers tell you the strength grade.

    Save Step 1: Look at the head: raised or stamped lines and numbers tell you the strength grade.

  2. Three radial lines mean SAE Grade 5; six radial lines mean SAE Grade 8; no lines usually means Grade 2 or ungraded.

    Save Step 2: Three radial lines mean SAE Grade 5; six radial lines mean SAE Grade 8; no lines usually means Grade 2 or ungraded.

  3. Metric bolts are stamped with property classes — 8.8, 10.9, and 12.9 — beside the manufacturer mark.

    Save Step 3: Metric bolts are stamped with property classes — 8.8, 10.9, and 12.9 — beside the manufacturer mark.

  4. Stainless bolts show an A2 or A4 steel group plus a strength number, such as A2-70 or A4-80.

    Save Step 4: Stainless bolts show an A2 or A4 steel group plus a strength number, such as A2-70 or A4-80.

  5. Letters or symbols identify the manufacturer — useful when tracing a defective batch.

    Save Step 5: Letters or symbols identify the manufacturer — useful when tracing a defective batch.

  6. Match the bolt grade to the load, and the nut grade to the bolt — never mix grades in a safety-critical joint.

    Save Step 6: Match the bolt grade to the load, and the nut grade to the bolt — never mix grades in a safety-critical joint.

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