The marks on a bolt head tell you its grade (strength). Nuts should be the same grade as the bolt or stronger.
Inch bolts (SAE J429)
| Grade | Head marking | Material | Min. tensile strength |
|---|---|---|---|
| Grade 2 | No marks | Low or medium carbon steel | 74,000 psi (1/4" to 3/4"), 60,000 psi (over 3/4" to 1-1/2") |
| Grade 5 | 3 radial lines | Medium carbon steel, quenched and tempered | 120,000 psi (1/4" to 1"), 105,000 psi (over 1" to 1-1/2") |
| Grade 8 | 6 radial lines | Medium carbon alloy steel, quenched and tempered | 150,000 psi (1/4" to 1-1/2") |
Metric bolts (ISO 898-1)
Metric bolts are stamped with their property class.
| Class | Head marking | Min. tensile strength | Comparable to |
|---|---|---|---|
| 8.8 | "8.8" | 800 MPa (830 MPa above M16) | Grade 5 |
| 10.9 | "10.9" | 1,040 MPa | Grade 8 |
| 12.9 | "12.9" | 1,220 MPa | Socket head cap screws |
The first number times 100 is the tensile strength in MPa; the second number is the yield strength as a fraction of tensile (8.8 yields at 80%).
Stainless steel
| Marking | Material | Notes |
|---|---|---|
| "18-8" or no grade mark | 18% chromium, 8% nickel (304 family) | Good corrosion resistance, general use |
| "316" | 316 stainless | Better resistance to salt water and chemicals |
| "A2-70" | Metric 304 family | 700 MPa tensile |
| "A4-70" / "A4-80" | Metric 316 family | 700 / 800 MPa tensile |
Nuts
| Nut | Marking |
|---|---|
| Grade 2 / Grade A | No marks |
| Grade 5 / Grade B | 3 dots or lines, 120° apart |
| Grade 8 / Grade C | 6 dots or lines, 60° apart |
| Metric class 8 or 10 | "8" or "10" on the face or flat |