Stainless Steel A2 Carriage Bolts with Flange Nuts — M3 to M12 Cup Square Coach Screws Set

A general rule is to add the thickness of all materials being joined, then add approximately 1–1.5× the bolt diameter for thread engagement in the nut. For example, joining two 10mm boards with an M6 bolt typically calls for a 25mm or 30mm length (20mm material + ~6–9mm thread). When in doubt, size up — you can always add a washer to take up slack. Length conversions: 25mm ≈ 1", 50mm ≈ 2", 75mm ≈ 3", 100mm ≈ 4".

A2 stainless steel (also called 304 stainless) is an austenitic grade that offers good corrosion resistance and a tensile strength typically around 500–700 MPa depending on size and processing. It handles most indoor and sheltered outdoor loads well. For highly aggressive marine or chemical environments, A4 (316) stainless is generally recommended for longer service life.

Yes. A2 stainless steel provides solid rust and corrosion resistance for most outdoor uses — decking, fencing, garden furniture, and pergolas are common applications. For direct saltwater or coastal splash-zone exposure, A4 stainless is generally a better choice for long-term performance. Avoid prolonged contact with dissimilar metals (e.g., bare aluminum) to prevent galvanic corrosion.

These are metric fasteners following the ISO/DIN coarse-thread pitch standard (e.g., M6 = 1.0mm pitch, M8 = 1.25mm pitch, M10 = 1.5mm pitch). They are not compatible with UNC or UNF imperial threads. Always confirm your existing hardware is metric before ordering. If you're unsure, contact us with your application details.

Each set includes one carriage bolt and one matching flange nut of the same diameter and grade — no separate nut purchase needed. The flange nut's built-in washer face distributes load and eliminates the need for a separate flat washer in most applications. Choose 1 Set for single replacements or 10 Sets for larger builds.

Yes — the cup square (carriage bolt) design features a domed head and a square shank section directly beneath it. When driven into wood or a pre-drilled square hole, the shank bites into the material and prevents the bolt from rotating while you tighten the nut from the other side. This makes single-person installation practical for most woodworking and deck construction scenarios. For very hard materials, a square-punch or chisel can help seat the shank.

In most cases, no. The flange nut has an integrated wide-diameter bearing face that acts like a built-in washer, spreading the clamping load over a larger surface area and reducing the chance of the nut pulling through softer wood. However, for very soft materials or oversized holes, adding a flat washer under the flange nut can provide extra bearing surface and is generally a good practice.

Divide the millimeter value by 25.4 to get decimal inches. Quick reference chart:

• 8mm ≈ 5/16"
• 10mm ≈ 3/8"
• 16mm ≈ 5/8"
• 20mm ≈ 13/16"
• 25mm ≈ 1"
• 30mm ≈ 1-3/16"
• 40mm ≈ 1-9/16"
• 50mm ≈ 2"
• 75mm ≈ 3"
• 100mm ≈ 4"
• 120mm ≈ 4-3/4"