Hex bolts
A hex bolt has a six-sided head driven by a wrench or socket. It is the most common bolt type for structural and mechanical assemblies. Always use with a nut, or thread into a tapped hole.
SAE/ASTM strength grades (imperial)
| Grade | Head marking | Tensile strength | Use |
|---|---|---|---|
| Grade 2 | No marks (blank) | 74,000 psi | General low-load applications |
| Grade 5 | 3 radial lines | 120,000 psi | Automotive, machinery, general structural |
| Grade 8 | 6 radial lines | 150,000 psi | High-load structural, suspension, engine components |
Metric property classes
| Class | Head marking | Approx. tensile | Equivalent SAE grade |
|---|---|---|---|
| 8.8 | "8.8" | 116,000 psi | ~Grade 5 |
| 10.9 | "10.9" | 145,000 psi | ~Grade 8 |
| 12.9 | "12.9" | 174,000 psi | Above Grade 8 |
Common imperial thread sizes
| Size | Thread (UNC) | Wrench size (hex) |
|---|---|---|
| 1/4" | 1/4"-20 | 7/16" |
| 5/16" | 5/16"-18 | 1/2" |
| 3/8" | 3/8"-16 | 9/16" |
| 1/2" | 1/2"-13 | 3/4" |
| 5/8" | 5/8"-11 | 15/16" |
| 3/4" | 3/4"-10 | 1-1/8" |
Common metric thread sizes
| Size | Thread pitch (coarse) | Wrench size |
|---|---|---|
| M6 | 1.0 mm | 10 mm |
| M8 | 1.25 mm | 13 mm |
| M10 | 1.5 mm | 17 mm |
| M12 | 1.75 mm | 19 mm |
| M16 | 2.0 mm | 24 mm |
Zinc-plated vs hot-dip galvanized vs stainless
- Zinc-plated — indoor use; moderate short-term corrosion resistance
- Hot-dip galvanized (HDG) — outdoor structural use, decks, bridges; thick zinc coat
- Stainless steel (304/316) — marine, food service, outdoor decorative; no rust but lower strength than Grade 8
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