Bolt Head Markings
How to read grade 8 bolt markings, metric bolt markings, and use a bolt identification chart for strength and material.
What bolt head markings mean
Head markings on bolts indicate strength grade and sometimes manufacturer. SAE (imperial) grades use radial lines or letters; metric bolts use property class numbers such as 8.8 or 10.9. Knowing these helps you match or upgrade fasteners correctly.
Bolt grade identification chart
| SAE grade | Markings | Typical use |
|---|---|---|
| Grade 2 | None or minimal | General purpose |
| Grade 5 | 3 radial lines | Automotive, machinery |
| Grade 8 | 6 radial lines | High strength, suspension |
Metric bolt markings
Metric property classes are stamped on the head (e.g. 8.8, 10.9). The first number relates to tensile strength; the second to yield ratio. Use our bolt torque calculator with the correct grade for safe tightening.
| Property class | Approx. tensile (MPa) |
|---|---|
| 4.6 | 400 |
| 8.8 | 800 |
| 10.9 | 1040 |
| 12.9 | 1220 |
FAQ
What do bolt head markings mean?
They indicate strength grade. SAE uses radial lines (e.g. six lines = Grade 8). Metric uses numbers like 8.8 or 10.9 for property class.
How do you identify grade 8 bolt markings?
Grade 8 bolts have six radial lines on the head. They are high-strength alloy steel, common in automotive and structural applications.
Are metric bolt markings different from SAE?
Yes. Metric uses property class numbers (8.8, 10.9) on the head. SAE uses grade symbols like lines or letters (Grade 5, Grade 8).