M8 M10 M12 Carriage Bolts – Cup Square Domed Head, Black A2-70 Stainless Steel

Generally, the bolt length should equal the total thickness of the materials being joined, plus enough thread engagement for the nut — typically at least 1× the bolt diameter. For example, joining two 20mm boards with an M8 bolt would typically require at least a 50mm (≈2") bolt to allow for the nut and washer. When in doubt, size up by one increment.

These bolts are manufactured to the A2-70 specification, which generally indicates a minimum tensile strength of 700 MPa and a proof load suitable for most structural and general-purpose fastening applications. This makes them appropriate for deck construction, timber framing, and similar load-bearing uses.

A2 stainless steel offers good corrosion resistance and is generally suitable for outdoor environments including garden structures, decking, and fencing. For highly aggressive marine or coastal environments where salt spray is constant, A4 (316) stainless steel is typically recommended for longer service life.

These carriage bolts use standard ISO metric coarse thread (M8, M10, or M12). They are compatible with any standard metric hex nut and flat washer of the matching diameter. No special or proprietary hardware is required.

The black finish is typically a black oxide or passivation treatment applied over the A2 stainless steel base. It provides a dark aesthetic appearance while the underlying stainless steel maintains its inherent corrosion resistance. In most cases, minor surface scratches will not compromise the bolt's rust resistance since the stainless steel substrate remains intact.

No special tools are required. The square shank under the domed head is designed to embed into wood or soft material when the nut is tightened, preventing the bolt from spinning. You only need a standard wrench or socket on the nut side. This makes them particularly convenient for single-person installation in timber applications.

Carriage bolts are primarily designed for wood-to-wood or wood-to-metal connections where the square neck can grip and embed into the softer material. For metal-to-metal applications, a standard hex bolt is generally more appropriate since the square neck may not seat properly in hard materials and could spin freely. For composite decking, check whether the material is soft enough to allow the neck to seat — in most cases it will work, but a test fit is recommended.