Bolt grade markings
Count the radial lines on the head: none is Grade 2, three is Grade 5 and six is Grade 8. Metric bolts have their property class stamped on the head, such as 8.8, 10.9 or 12.9.
Sources checked
SAE bolts: count the lines
Inch bolts made to SAE J429 show their grade as raised or stamped lines running out from the center of the head. No lines is Grade 2, three lines is Grade 5 and six lines is Grade 8. Grade 2 is plain low or medium carbon steel; Grades 5 and 8 are heat treated (quenched and tempered), which is where their extra strength comes from.
Heads often carry the maker’s initials or logo as well. Those say who made the bolt, not how strong it is.
Metric bolts: read the number
Metric bolts made to ISO 898-1 have their property class stamped on the head: 4.6, 8.8, 10.9 or 12.9 are the common ones. The number is a code. The part before the point is a hundredth of the nominal tensile strength in megapascals, and the part after it is the yield strength as a share of that. Class 8.8 means 800 MPa and 80%, so a yield strength of about 640 MPa.
Strength by grade
| Grade | Head marking | Material | Proof stress | Min. tensile strength |
|---|---|---|---|---|
| Grade 2 | No marks on the head | Low or medium carbon steel | 55,000 psi up to 3/4"; 33,000 psi above | 74,000 psi up to 3/4"; 60,000 psi above |
| Grade 5 | 3 radial lines on the head | Medium carbon steel, quenched and tempered | 85,000 psi up to 1"; 74,000 psi above | 120,000 psi up to 1"; 105,000 psi above |
| Grade 8 | 6 radial lines on the head | Medium carbon alloy steel, quenched and tempered | 120,000 psi | 150,000 psi |
| Class 4.6 | "4.6" on the head | Low or medium carbon steel | 225 MPa | 400 MPa |
| Class 8.8 | "8.8" on the head | Medium carbon steel, quenched and tempered | 580 MPa up to M16; 600 MPa above | 800 MPa up to M16; 830 MPa above |
| Class 10.9 | "10.9" on the head | Alloy steel, quenched and tempered | 830 MPa | 1,040 MPa |
| Class 12.9 | "12.9" on the head | Alloy steel, quenched and tempered | 970 MPa | 1,220 MPa |
Proof stress is the most a bolt can carry without stretching for good; the torque charts tighten to 75% of it. SAE J429 values from Portland Bolt and Fastenal; ISO 898-1 values from Fastenal.
What the grade does to torque
A stronger bolt can be tightened to a higher clamp load, so it takes more torque. Dry, at 75% of proof load, a 1/2-13 bolt takes 48.8 ft-lb in Grade 2, 75.4 ft-lb in Grade 5 and 106 ft-lb in Grade 8. Using a Grade 8 value on a Grade 2 bolt would stretch it well past its proof load.
Every size and grade is in the SAE chart and the metric chart, and the calculator shows the working for any one bolt.
Questions
How many lines does a Grade 8 bolt have?
Six radial lines on the head. Grade 5 has three, and Grade 2 has none.
What does 10.9 on a bolt mean?
It’s the ISO 898-1 property class. The 10 stands for a nominal tensile strength of 1,000 MPa and the .9 for a yield strength of 90% of that. The standard’s minimum tensile strength for class 10.9 is 1,040 MPa, and its proof stress 830 MPa.
Is Grade 8 the same as class 10.9?
Close in strength, not interchangeable. Grade 8’s proof stress of 120,000 psi is 827 MPa, against 830 MPa for 10.9; Grade 5 (586 MPa) and class 8.8 (580 MPa) pair up the same way. But inch and metric threads don’t fit each other, so a bolt has to be replaced with its own kind.
What grade is a bolt with no markings?
An SAE bolt with no lines is Grade 2 or lower. If a bolt has no marking at all you can’t know its strength, so use the lowest grade’s torque or replace it with a marked bolt.
Sources
- Bolt Depot: Bolt grade markings and strength chart (checked October 14, 2026)
- Portland Bolt: SAE J429: proof load, yield and tensile strength for Grades 2, 5 and 8 (checked October 14, 2026)
- Fastenal: Mechanical properties of inch fasteners, with UNC and UNF tensile stress areas (checked October 14, 2026)
- Fastenal: Mechanical properties per ISO 898-1 (externally threaded fasteners): proof stress by property class (checked October 14, 2026)
Spotted a number that doesn’t match your manual or a source? Tell us; corrections are logged on the sources page.