M3 M4 M5 M6 Carriage Bolts – A2-70 Stainless Steel Cup Square Domed Screws

Generally, the bolt length should equal the total thickness of all materials being joined, plus enough thread engagement for the nut — typically at least 1.5× the bolt diameter. For example, joining two 20mm (≈3/4") boards with an M6 bolt would typically require at least a 50mm (≈2") bolt. When in doubt, size up to the next available length.

A2 refers to the austenitic stainless steel grade (equivalent to 304 stainless), and 70 indicates a minimum tensile strength of 700 MPa. This makes them suitable for general structural fastening in most indoor and moderate outdoor environments. For higher-load applications, consult an engineer to confirm suitability.

A2 stainless steel offers good corrosion resistance and is generally suitable for outdoor use in most climates, including rain-exposed decks and garden structures. For highly corrosive environments — such as coastal or marine applications with direct saltwater exposure — A4 (316) stainless steel is typically recommended instead.

These bolts use standard ISO metric coarse threads (DIN 603 / ISO 8677 profile). M3 through M6 sizes are compatible with standard metric hex nuts and flat washers of the corresponding size, available at any hardware store. They are not compatible with UNC or UNF inch-thread hardware.

No special tools are required on the bolt side. The domed head and square shank are designed to be inserted into a pre-drilled hole — the square neck bites into the material to prevent rotation while you tighten the nut from the other side with a standard wrench or socket. Ensure the hole diameter matches the bolt shank (not the square neck) for a proper fit.

For light-duty wood joints such as furniture or shelving, M3–M4 bolts are generally sufficient. For structural wood connections such as deck framing, pergola beams, or fence posts, M5–M6 bolts are typically recommended. Always match the bolt diameter to the pre-drilled hole size for a snug, secure fit.

Yes. Carriage bolts can be used with thin metal plates, composite panels, and engineered wood products. For metal applications, ensure the hole is sized to allow the square neck to seat properly — in harder materials, a square-hole punch or careful step-drilling may be needed to achieve the anti-rotation grip.