M4–M12 Carriage Bolts A2 Stainless Steel | Cup Square Neck Screws

A general rule is to add the total thickness of all materials being joined, then add at least one full nut height plus a few threads of engagement. For example, joining two 20mm (≈3/4") boards typically calls for a bolt of at least 50mm (≈2"). When in doubt, size up — a bolt that is slightly too long can be fitted with extra washers or a coupling nut.

A2 stainless steel (ISO 3506 property class 70) generally provides a minimum tensile strength of 700 MPa and a proof load stress of approximately 450 MPa. Actual load capacity depends on bolt diameter, thread engagement length, and the materials being fastened. For structural or load-bearing applications, always consult a qualified engineer.

A2 stainless steel offers good corrosion resistance and is suitable for most outdoor applications including garden furniture, decking, fencing, and playground equipment. For highly aggressive environments with prolonged saltwater or chemical exposure — such as coastal marine installations — A4 (316) stainless steel is generally recommended for superior chloride resistance.

These bolts use standard ISO metric coarse threads conforming to DIN 603 / ISO 8677. Thread pitch varies by diameter: M4 = 0.7mm, M5 = 0.8mm, M6 = 1.0mm, M8 = 1.25mm, M10 = 1.5mm, M12 = 1.75mm. They are fully compatible with standard metric hex nuts, nyloc (prevailing torque) nuts, and flat washers available from any hardware supplier.

The square section directly beneath the domed head bites into the wood grain or seats into a square-punched hole in metal sheet as the nut is tightened. This mechanical lock prevents rotation without requiring a wrench or screwdriver on the head side — ideal for connections where one face is inaccessible or where a clean, tool-mark-free surface is required.

For light timber furniture and shelving, M6 or M8 bolts are typically sufficient. For heavier structural connections such as pergola posts, deck ledger boards, or load-bearing beams, M10 or M12 is generally preferred. Always verify load requirements against your local building code and consult a structural engineer for critical applications.

A flat washer under the nut is strongly recommended in most applications. It distributes the clamping load over a larger surface area, reduces the risk of the nut pulling through softer materials like wood or plastic sheeting, and makes future disassembly easier. For outdoor use, choose a washer in the same A2 stainless steel grade to avoid galvanic corrosion.