Black Carriage Bolts Grade 8.8 | M6–M24 Cup Square Domed Head Screws

A general rule: add the thickness of all materials being joined, then add approximately 1–2 thread pitches beyond the nut. For example, joining two 10mm boards with an M8 bolt typically requires at least 25–30mm (about 1–1.2"). Always measure your stack-up before ordering. Our variants range from 20mm (≈0.79") to 150mm (≈5.91").

Grade 8.8 is a medium-high tensile strength classification under ISO 898-1. It indicates a minimum tensile strength of 800 MPa and a yield strength of 640 MPa, making these bolts suitable for structural and load-bearing applications where standard hardware-grade fasteners would be insufficient.

These bolts feature a black oxide finish, which provides mild corrosion resistance and is generally suitable for indoor or sheltered outdoor use. For prolonged outdoor exposure or high-humidity environments, we recommend applying a protective coating or sealant after installation, or contacting us to confirm suitability for your specific conditions.

These bolts use standard ISO metric coarse threads (M6 through M24), which are compatible with standard metric nuts and threaded inserts. They are not compatible with UNC/UNF inch-standard fasteners. Always verify thread pitch before pairing with existing hardware.

Full thread bolts are threaded along the entire shank length, providing maximum grip in thinner materials or when threading into a nut close to the head. Part thread bolts have a smooth shank section near the head — this unthreaded portion typically provides better shear resistance and is preferred for thicker assemblies where the bolt passes through a clearance hole before engaging the nut.

Drill a clearance hole matching the bolt diameter through your material. Insert the bolt from the front (domed head side). The square neck under the head will bite into wood or soft material to prevent rotation. Apply a washer and nut on the back side, then tighten with a wrench. For metal or hard materials, a square hole or anti-rotation plate may be needed.

For typical deck or fence rail connections using 2x lumber (≈38mm/1.5" thick), M10 or M12 bolts in the 80–100mm range are generally appropriate. For post-to-beam connections, M12–M16 in 120–150mm lengths are more commonly used. Always consult local building codes for structural fastener requirements.