Grade 5 vs. Grade 8 Bolts
Grade 8 is stronger but more brittle and more expensive. Strength alone does not make a better joint — matching the grade to the load does.
| Feature | Grade 5 Bolt | Grade 8 Bolt |
|---|---|---|
| Head marking | 3 radial lines | 6 radial lines |
| Proof strength | 85,000 psi | 120,000 psi |
| Tensile strength | 120,000 psi | 150,000 psi |
| Material | Medium carbon steel, Q&T | Medium carbon alloy steel, Q&T |
| Best for | Automotive, general machinery | High-clamp, high-stress joints |
Verdict
Use Grade 5 for most automotive and machinery work. Step up to Grade 8 only where a higher clamp load is specified — never substitute strength for proper joint design.