Square Head Bolt
Bolts · square bolt · square head machine bolt
Definition: A square head bolt has a four-sided head driven by an open-end wrench — the classic pre-1900 bolt still used on old machinery and restoration work.
Anatomy
■ Head■ Shank■ Threads■ Point
Governing standards: ASME B18.2.1
Common Sizes
| Inch | Metric | Head width | Head height |
|---|---|---|---|
| 1/4-20 | M6 | 7/16 | — |
| 3/8-16 | M10 | 9/16 | — |
| 1/2-13 | M12 | 3/4 | — |
Representative sizes. Full dimensional data lives in the standards listed below.
Grades & Strength
| Grade | Material | Tensile (psi) | Marking |
|---|---|---|---|
| Grade 2 | low carbon steel | 74,000 | None |
| Grade 5 | medium carbon steel, Q&T | 120,000 | 3 radial lines |
Materials
- Zinc-plated steel
- Plain steel
- Stainless steel
Pros & Cons
Pros
- Authentic for restorations
- Big wrench flats resist rounding
- Strong square drive
Cons
- Needs room to swing a wrench
- Fewer wrench sizes fit
- Not stocked everywhere
Typical Uses
- Antique machinery
- Farm equipment
- Restoration projects
- Decorative ironwork
Alternatives & Related
- Hex Bolt — Modern standard with more wrench access
- Carriage Bolt — Square neck bolt for wood
Where to Buy
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FAQ
Where are square head bolts used?
Mostly on antique machinery, farm implements, and restoration work where the original square-head look is wanted.
Can I replace a square head bolt with a hex bolt?
Functionally yes, but on restorations the square head is part of the period-correct appearance — replace in kind if authenticity matters.