M6 M8 A2 304 Stainless Steel Carriage Bolts — Cup Square Coach Bolts, Full Thread, 12mm–60mm

Add up the total thickness of all materials being joined, then add at least 10–15mm (roughly 3/8"–5/8") to accommodate the nut and any washer. For example, joining two 20mm (3/4") boards typically calls for a 50mm (2") bolt. When in doubt, size up — a slightly longer bolt can be accommodated with an extra washer, while a bolt that's too short won't allow the nut to engage fully.

A2 304 stainless steel carriage bolts at A2-70 grade generally have a tensile strength of approximately 500–700 MPa. M6 bolts typically handle proof loads around 4.6 kN, while M8 bolts generally handle around 8.0 kN. For structural or load-bearing applications, always verify requirements with a qualified engineer and check your local building code.

Yes — A2 304 stainless steel offers strong resistance to rust, oxidation, and most atmospheric corrosion, making these bolts well-suited for outdoor decking, fencing, pergolas, and garden structures. For highly aggressive marine or coastal environments with constant saltwater exposure, A4 316 stainless steel is generally recommended for maximum long-term corrosion resistance.

These bolts use the standard ISO metric coarse thread — M6 at 1.0mm pitch and M8 at 1.25mm pitch. They are compatible with any standard ISO metric hex nuts, flange nuts, or wing nuts of the corresponding size. They are not compatible with UNC or UNF imperial-threaded nuts. For a fully corrosion-resistant assembly, pair with A2 stainless steel nuts and flat washers.

A carriage bolt (also called a coach bolt or cup square bolt) has a smooth, domed head with a square shank section directly beneath it. When inserted into a pre-drilled hole, the square section bites into the material and prevents the bolt from spinning — so you only need a wrench on the nut side. This makes them ideal for wood-to-wood and wood-to-metal connections where one face must remain flush and snag-free, or where the bolt head side is inaccessible during tightening.

Using a flat washer under the nut is generally recommended, especially in softer woods or composite decking materials, to distribute the clamping load and prevent the nut from pulling through the surface. The domed head side typically does not require a washer since it sits flush against the workpiece. Washers are sold separately — contact us if you need matching A2 stainless flat washers.

Yes. A2 304 stainless steel is generally compatible with modern ACQ (Alkaline Copper Quaternary) and CA (Copper Azole) pressure-treated lumber commonly used for decking and fencing. Older CCA-treated wood and some high-retention treatments can be more chemically aggressive; in those cases, A4 316 stainless steel is the safer long-term choice. Always check the lumber manufacturer's fastener compatibility recommendations for your specific treatment type and retention level.