Bolt grade markings and strength chart
Count the radial lines on an inch bolt head: none for Grade 2, three for Grade 5, six for Grade 8. Metric bolts carry the property class, such as 8.8 or 10.9, as a number. By strength, Grade 5 matches class 8.8 and Grade 8 matches class 10.9.
Contents7 sections
Key takeaways
- SAE J429: no mark = Grade 1 or 2, three lines = Grade 5, six lines = Grade 8 1.
- Metric heads carry the class symbol plus the maker's mark 2.
- Grade 5 is within about 3% of class 8.8 in strength, and Grade 8 within about 1% of class 10.9. Class 12.9 has no SAE J429 equal.
- Use a nut whose proof strength is at least the bolt's tensile strength 1.
SAE J429 head markings
| Grade | Head | Marking |
|---|---|---|
| Grade 2 | No grade mark | |
| Grade 5 | 3 radial lines, 120° apart | |
| Grade 8 | 6 radial lines, 60° apart |
Markings per SAE J429 as shown by Fastenal1.
ISO 898-1 property class markings
| Class | Head | Marking |
|---|---|---|
| Class 4.6 | “4.6” stamped on the head with the maker’s mark | |
| Class 8.8 | “8.8” stamped on the head with the maker’s mark | |
| Class 10.9 | “10.9” stamped on the head with the maker’s mark | |
| Class 12.9 | “12.9” stamped on the head with the maker’s mark |
ISO 898-1 requires the property class symbol on the head2.
How to identify the grade
Inch bolts (SAE J429). Count the radial lines on the head. Grade 5 has three lines 120° apart and Grade 8 has six lines 60° apart. Grades 1 and 2 both have no grade mark 1. A plain head therefore tells you only that the bolt is not marked as Grade 5 or 8. Grade 1, at 33 ksi proof, and Grade 2 look the same. Grade 8.2, a low-carbon boron steel with Grade 8 strength, uses a different radial pattern 1.
Metric bolts (ISO 898-1). The head carries the property class symbol and the manufacturer’s identification mark 2. The first number is the nominal tensile strength in hundreds of MPa. The number after the dot is ten times the ratio of yield to tensile. So 8.8 means 800 MPa nominal tensile with yield at 0.8 of that. A leading zero, as in 08.8, marks a bolt with reduced loadability 2.
Structural bolts. Heavy hex structural bolts are stamped with the grade, “A325” or “A490”, plus the maker’s ID. Twist-off assemblies are stamped “F1852” or “F2280” 3. ASTM F3125 now covers all four 4. See structural bolting for their pretension rules.
For a step-by-step check, use the bolt grade identification tool.
SAE vs metric by strength
Converted at 1 ksi = 6.894757 MPa 5:
| SAE grade (size) | Proof | Tensile | Nearest class |
|---|---|---|---|
| Grade 2 (1/4–3/4 in) | 55 ksi (379 MPa) | 74 ksi (510 MPa) | Between 4.6 and 8.8 |
| Grade 2 (7/8–1-1/2 in) | 33 ksi (228 MPa) | 60 ksi (414 MPa) | 4.6 (225 / 400 MPa) |
| Grade 5 (1/4–1 in) | 85 ksi (586 MPa) | 120 ksi (827 MPa) | 8.8 (580–600 / 800–830 MPa) |
| Grade 8 (1/4–1-1/2 in) | 120 ksi (827 MPa) | 150 ksi (1,034 MPa) | 10.9 (830 / 1,040 MPa) |
Scroll the table sideways to see every column.
SAE values are per SAE J429 1 and metric values per ISO 898-1 2. Class 12.9 has a 970 MPa (141 ksi) proof stress, which is above every SAE J429 grade listed here. Equal strength does not make the parts interchangeable, because inch and metric threads do not fit each other.
Matching nuts
The rule is to pick a nut whose minimum proof strength is at least the bolt’s minimum tensile strength 1.
- Grade 2 bolt: a low-carbon nut. SAE J995 Grade 5 or 8 nuts may substitute 1.
- Grade 5 bolt: a J995 Grade 5 nut, with a 120 ksi proof on UNC threads up to 1 in. Grade 8 may substitute 1.
- Grade 8 bolt: a J995 Grade 8 nut, with a 150 ksi proof, which equals the bolt’s tensile strength 1.
- Metric: the older ISO 898-2 table gives class 8 nuts 800–920 MPa proof and class 10 nuts 1,040–1,060 MPa, which covers 8.8 and 10.9 bolts 1. Class 12 nuts are listed at 1,140–1,200 MPa. That is just under the 1,220 MPa tensile strength of 12.9 bolts, so check the current ISO 898-2 or the bolt maker’s pairing.
- Galvanized structural assemblies: only ASTM A563 DH/DH3 or A194 2H nuts 3.
Sources
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Spotted a value that doesn’t match its source? Report an error
Related
- Grade 5Grade 5 bolt torque from 1/4 in to 1 in, dry, zinc-plated and lubricated, with SAE J429 strength, the three-line head mark and the step down to 74 ksi above 1 in.
- Grade 8Grade 8 bolt torque from 1/4 in to 1 in, dry, zinc-plated and lubricated, with SAE J429 proof, yield and tensile strength and the six-line head marking.
- Class 8.8Class 8.8 bolt torque from M3 to M30, dry, zinc-plated and lubricated, with the ISO 898-1 proof stress step from 580 MPa to 600 MPa above M16.
- Class 10.9Class 10.9 bolt torque from M3 to M30, dry, zinc-plated and lubricated, with ISO 898-1 strength (830 MPa proof) and how it compares with 8.8 and 12.9.