M4–M12 Cup Square Carriage Bolts with Hex Full Nuts | Zinc Plated | Sets of 2–50

Generally, the bolt length should equal the total thickness of all materials being joined, plus enough thread engagement for the nut — typically at least 1× the bolt diameter beyond the last material layer. For example, joining two 10mm boards with an M6 bolt would typically require at least 26mm (20mm material + 6mm thread). When in doubt, size up by one increment.

Zinc-plated carriage bolts are generally manufactured to a Grade 4.8 or equivalent standard, which typically provides a tensile strength of around 400 MPa. Actual load capacity depends on bolt diameter, material thickness, and joint design. For structural or load-critical applications, please confirm the grade with us before ordering.

The zinc electroplating provides moderate corrosion resistance suitable for indoor use and occasional moisture exposure. For prolonged outdoor exposure, high humidity, or direct contact with treated lumber, a hot-dip galvanized or stainless steel fastener is generally recommended. Contact us to confirm suitability for your specific environment.

These bolts use standard ISO metric coarse thread (M4 through M12). The included hex nuts are matched to the same metric coarse pitch, so no additional hardware is needed for a complete fastening assembly. They are not compatible with UNC or UNF inch-thread hardware.

The cup square (carriage bolt) design features a smooth domed head and a square shank section directly beneath it. When driven into wood or a square-punched hole, the square shoulder bites into the material and prevents the bolt from spinning while you tighten the nut — no wrench needed on the bolt head side.

Each listing is sold as a complete set that includes both the carriage bolts and matching hex full nuts. Pack quantities vary by size: smaller diameters (M4–M5) are available in sets of 20 or 50, while larger diameters (M8–M12) come in sets of 2, 5, or 10. Select your preferred size and quantity combination from the variant options above.

Carriage bolts are most commonly used with wood, but they can also be used with metal if the receiving hole is square-punched or if a backing plate is used to prevent rotation. For metal-to-metal applications without a square hole, a standard hex bolt is generally a better choice. Contact us if you are unsure which fastener type suits your application.