The marks stamped on a bolt head are not decoration. They tell you the strength class, and sometimes the manufacturer. Reading them takes ten seconds and prevents putting a Grade 2 bolt where a Grade 8 belongs.
Imperial bolts: count the lines
SAE grades are marked with radial lines stamped on the head. The rule is simple: number of lines + 2 = grade.
| Marking | Grade | Material | Tensile | Yield | Typical use |
|---|---|---|---|---|---|
| No lines | SAE Grade 2 | Low carbon steel | 74 ksi | 57 ksi | General, non-structural |
| 3 lines | SAE Grade 5 | Medium carbon, quenched and tempered | 120 ksi | 92 ksi | Automotive, machinery |
| 6 lines | SAE Grade 8 | Medium carbon alloy, quenched and tempered | 150 ksi | 130 ksi | Suspension, high load |
| 6 lines, often black | ASTM A574 | Alloy steel socket cap | 180 ksi | — | Tooling, precision assemblies |
| A2 / A4 stamp | Stainless | 304 / 316 | 70–80 ksi | — | Corrosion resistance, not strength |
Metric bolts: read the number
Metric property classes are stamped as two numbers separated by a dot, such as 8.8. Both halves mean something:
- The first number × 100 is the nominal tensile strength in MPa. So 8.8 means 800 MPa.
- The second number is the ratio of yield to tensile, as tenths. So 8.8 means yield is 80% of 800, that is 640 MPa.
| Marking | Tensile | Yield | Equivalent in feel |
|---|---|---|---|
| 4.6 | 400 MPa | 240 MPa | close to Grade 2 |
| 4.8 | 400 MPa | 320 MPa | — |
| 8.8 | 800 MPa | 640 MPa | close to Grade 5 |
| 10.9 | 1040 MPa | 940 MPa | close to Grade 8 |
| 12.9 | 1220 MPa | 1100 MPa | above Grade 8 |
Where the marks sit
- Hex bolts carry the grade on the top face of the head, usually alongside a maker mark.
- Socket cap screws often have no marks at all; assume ASTM A574 unless the packaging says otherwise, and treat unmarked ones as unknown.
- Nuts use their own system. Metric nuts show a single number: 8 pairs with class 8.8, 10 with 10.9. Imperial nuts use dots or lines on the flats.
- Stainless shows A2 or A4 plus a strength number, such as A2-70. This is corrosion class first, strength second.
Three things worth knowing
Grade 8 is not always the right answer. Higher grade means higher strength but lower ductility. In a joint that sees vibration and shock, a Grade 5 bolt that stretches slightly can outlast a Grade 8 that snaps.
Stainless is not high strength. A4-80 tops out around 800 MPa, below class 10.9. Choosing stainless is a corrosion decision, and you pay for it in strength.
No marks means no guarantee. An unmarked bolt in a load path is an unknown. Replace it rather than assume.
Related: standards cross-reference and metric to imperial conversion.