A bolt’s grade tells you its strength, and choosing the right one means matching strength to the load without over- or under-building the joint. Two systems dominate.
SAE (inch) grades
Marked by radial lines on the bolt head:
| SAE grade | Head marking | Relative strength |
|---|---|---|
| Grade 2 | No lines | Low (general use) |
| Grade 5 | 3 radial lines | Medium (automotive) |
| Grade 8 | 6 radial lines | High (machinery, suspension) |
Metric grades
Stamped as numbers like 8.8, 10.9, or 12.9 — higher numbers mean higher strength (8.8 ≈ SAE Grade 5, 10.9 ≈ Grade 8). Match the grade to the load and to what the assembly specifies, and use a nut of equal or higher grade so the threads don’t strip before the bolt reaches its rated load. Higher-grade bolts are stronger but more brittle, so they aren’t always right for shock or fatigue.
Frequently asked questions
What do the lines on a bolt head mean? They mark the SAE grade — three lines is Grade 5, six is Grade 8.
Is metric 10.9 strong? Yes — roughly equivalent to SAE Grade 8.
Can I mix bolt and nut grades? Use a nut of equal or higher grade than the bolt.