Technically reviewed by Vincent, Founder at HEXVARO
Last reviewed
Quick answer
Quick answer: common bolt head markings
Find the mark on the head, then read the grade. A marking helps you identify a bolt, but it is not proof of strength on its own.
| Mark on head | Meaning | Minimum tensile strength |
|---|---|---|
| 8.8 | Metric class 8.8 | 800 MPa up to M16, 830 MPa above |
| 10.9 | Metric class 10.9 | 1040 MPa |
| 12.9 | Metric class 12.9 | 1220 MPa |
| 3 radial lines | SAE Grade 5 | 120 ksi up to 1 in |
| 6 radial lines | SAE Grade 8 | 150 ksi |
Metric bolt markings
Metric hex bolts show the property class on the head. The maker's mark usually sits beside it. Read the first number × 100 for tensile strength in MPa. The second number gives the yield ratio, so 8.8 means about 80 % of tensile strength.
| Mark on head | Meaning | Minimum tensile strength |
|---|---|---|
| 4.8 | Metric class 4.8 | 400 MPa |
| 8.8 | Metric class 8.8 | 800 MPa up to M16, 830 MPa above |
| 10.9 | Metric class 10.9 | 1040 MPa |
| 12.9 | Metric class 12.9 | 1220 MPa |
SAE inch bolt markings
SAE J429 uses radial lines on the head. Count the lines to find the grade.
| Mark on head | Meaning | Minimum tensile strength |
|---|---|---|
| No grade mark | Usually SAE Grade 2 or similar low-carbon steel | 74 ksi up to 3/4 in |
| 3 radial lines | SAE Grade 5 | 120 ksi up to 1 in |
| 6 radial lines | SAE Grade 8 | 150 ksi |
ASTM and stainless markings
- A325 and A490: the grade appears on the head of a heavy hex structural bolt, with the maker's mark.
- B7: an ASTM A193 alloy steel stud grade for high-temperature and pressure service.
- B8 and B8M: ASTM A193 stainless grades, based on 304 and 316.
- A2-70 and A4-80: metric stainless marks. A2 or A4 is the steel group, and 70 or 80 is tensile strength divided by 10 in MPa.
Nut markings
Metric nuts may show a class number, such as 8 or 10. The nut class should match the bolt, so a class 8.8 bolt normally takes a class 8 nut, and a class 10.9 bolt takes a class 10 nut. Small nuts and low classes are often unmarked.
What a marking cannot prove
A head stamp shows what the maker declared. It does not prove the lot passed testing. For important work, ask for a material test certificate that links to the production lot, and test samples if the risk justifies it.
What to state in an RFQ
- Grade and the marking you expect on the head.
- Whether you need your own head mark, and the artwork.
- Material test certificates and the lot traceability you need.
- Standard, size, finish and quantity. MOQ is 500 pcs per specification.
Frequently asked questions
What do the numbers on a bolt head mean?
On metric bolts, they show the property class. For 8.8, the tensile strength is about 800 MPa and yield is about 80 % of that.
What grade is a bolt with 3 lines on the head?
Three radial lines mean SAE Grade 5.
What grade is a bolt with 6 lines on the head?
Six radial lines mean SAE Grade 8.
What does no marking on a bolt mean?
On an inch bolt, it usually means SAE Grade 2 or a similar low-carbon steel. Treat an unmarked bolt as low strength unless documents show otherwise.
What do the letters on a bolt head mean?
They are often the maker's mark. Letters and numbers together, such as A325 or B7, can show an ASTM grade.
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