Fastener Grades & Head Markings
The head marking on a bolt indicates its mechanical properties. Never replace a high-grade bolt with a lower-grade bolt, as it will lead to catastrophic failure under load.
| Grade / Class | Head Marking | Min. Tensile Strength | Material Base | Common Application |
|---|---|---|---|---|
| SAE Grade 2 | No Marks | 74,000 psi | Low Carbon Steel | Non-critical hardware, general woodworking. |
| SAE Grade 5 | 3 Radial Lines | 120,000 psi | Medium Carbon Steel | Automotive, machinery, moderate load. |
| SAE Grade 8 | 6 Radial Lines | 150,000 psi | Medium Carbon Alloy Steel | Heavy equipment, suspension, high stress. |
| Metric Class 8.8 | "8.8" | 800 MPa | Medium Carbon Steel | Equivalent to Grade 5. Common structural. |
| Metric Class 10.9 | "10.9" | 1000 MPa | Alloy Steel | Equivalent to Grade 8. High tension. |
| Metric Class 12.9 | "12.9" | 1200 MPa | Alloy Steel | Maximum commercial strength. |
How to Read Metric Classes
Metric property classes are designated by two numbers separated by a dot (e.g., 8.8).
- The first number is 1/100 of the nominal tensile strength in MPa. (8 * 100 = 800 MPa).
- The second number is 10 times the ratio of yield strength to tensile strength. (8 * 0.1 = 0.8, meaning Yield is 80% of Tensile).