SAE Grade 2
No lines. Low-carbon steel, general use.
≈ 74,000 psi
The thread, diameter, and — the part everyone actually looks up — the hex wrench size that fits each bolt, for both inch (UNC) and metric. Plus how to read the strength grade stamped on the head.
Wrench size is the distance across the flats of the head. Metric heads follow ISO; older DIN parts run one size larger on M10–M14 (noted where they differ).
The marks on a bolt head tell you its strength. Inch bolts (SAE) use radial lines; metric bolts (property class) use two stamped numbers. More marks / higher numbers = stronger.
| Bolt size ▲ | Thread (coarse) | Major Ø | Hex wrench (across flats) |
|---|
Coarse thread shown (UNC / metric coarse). Wrench = width across the flats of a standard hex head. Not the same as socket depth or thread size.
A 3/4" wrench fits a standard 1/2"-13 hex bolt head. The head is always bigger than the bolt's thread diameter — don't confuse the two.
13 mm for a standard ISO M8 hex head. (Some older DIN and flange variants differ, but 13 mm is the norm.) M6 = 10 mm, M10 = 16 mm ISO / 17 mm DIN, M12 = 18/19 mm.
Count the radial lines on an inch (SAE) bolt: none = Grade 2, three = Grade 5, six = Grade 8 — more lines, stronger. Metric bolts are stamped with two numbers like 8.8 or 10.9; higher is stronger. Never downgrade a bolt on a load-bearing joint.
Close. An 8.8 (800 MPa) is roughly equivalent to SAE Grade 5 (120 ksi), and 10.9 lines up with Grade 8. They're comparable in strength but not interchangeable in thread — never mix inch and metric threads.