What are square head bolts?
Square head bolts are machine bolts with a four-sided head that provides a secure wrench grip and a classic, period-correct look. The large bearing surface under the head helps distribute load, while the square shape resists rounding in high-torque installs. They are common in restoration work, timber framing, and industrial assemblies.
Why choose square head bolts
- High tool engagement — four flat sides give excellent wrench grip in tight or dirty environments.
- Traditional appearance — preferred for historic restorations and exposed architectural hardware.
- Robust clamping — large head flats and thick profiles tolerate higher installation torque.
- Outdoor-ready options — hot-dip galvanized and stainless selections for weather exposure.
Materials and finishes for square head bolts
- Hot-dip galvanized steel — thick zinc coating for durable exterior corrosion protection.
- Zinc-plated steel — economical finish for indoor or light outdoor use.
- Stainless steel 304 or 316 — broad corrosion resistance; 316 preferred near salt or chlorides.
- Pair with matching nuts and washers in the same material/finish for best corrosion compatibility.
Common applications for square head bolts
- Timber framing, barns, bridges, and utility structures
- Historic restoration and period-correct machinery
- Guardrails, walkways, and outdoor site furnishings
- Industrial equipment, fixtures, and heavy duty brackets
How to size and specify square head bolts
Order by diameter × thread × length (e.g., 5/8″-11 × 6″). Length is measured from under the head to the end of the bolt. Choose the thread series (UNC/UNF or metric), material/finish, and compatible nuts and washers to meet structural and corrosion requirements.
Installation tips for square head bolts
- Use flat washers to protect wood or painted steel and spread load; add lock washers or a locking nut where vibration exists.
- Keep tools fully seated on the flats to avoid corner damage; apply steady torque rather than impact if appearance matters.
- For exterior builds, keep hardware metals matched to reduce galvanic corrosion.