Black Stainless Steel Carriage Bolts M3–M12 | A2 Cup Square Dome Screws

Generally, the bolt length should equal the combined thickness of all materials being joined, plus enough thread engagement for the nut — typically at least 1× the bolt diameter beyond the last material. For example, joining two 10mm boards with an M6 bolt would typically require at least 26mm (≈1") of length. If in doubt, size up — a slightly longer bolt can be accommodated with an extra washer.

These bolts are made from A2 (304) stainless steel, which typically has a tensile strength of around 500–700 MPa depending on diameter and manufacturing tolerances. For structural or load-bearing applications, please verify the specific grade requirements with a qualified engineer before specifying these fasteners.

Yes. A2 stainless steel offers good corrosion resistance for most outdoor, garden, and moist environments — including decking, fencing, and pergola construction. For coastal or marine environments with high chloride exposure, A4 (316) stainless steel is generally recommended for longer service life. Contact us if you need marine-grade confirmation.

Yes. These bolts use standard ISO metric coarse threads conforming to the DIN 603 / ISO 8677 profile. M3 through M12 sizes are fully compatible with standard metric hex nuts, nylon lock nuts, and flat washers of the same diameter designation available at any hardware store in the US and internationally.

Carriage bolts feature a smooth domed head and a square neck that locks into wood grain when tightened, preventing rotation without needing a wrench on the bolt head. This makes them ideal for through-bolting timber, furniture, and decking where a clean, snag-free surface is needed — particularly where the bolt head is on a visible or touch-accessible face.

On the nut side, using a flat washer is generally recommended to distribute clamping load and protect the material surface from the nut bearing face. On the bolt-head side, the dome head typically seats directly into the material — no washer is needed there. For softer woods, a washer under the dome head can help prevent pull-through under heavy load.

Depending on the size selected, packing quantities range from 1 pc up to 100 pcs per order. Smaller sizes (M3–M4 in short lengths) are available in larger packs of 50 or 100, while larger sizes (M10–M12 in longer lengths) are typically sold in packs of 1–5. For a deck or fence project, count your bolt positions and add 10% for spares. For a repair job, a single-piece pack keeps costs low.