M3 M4 M5 Carriage Bolts with Hex Flange Nuts — A2 Stainless Steel

A general rule: bolt length should equal the total thickness of the materials being joined, plus typically 1–2 thread pitches beyond the nut. For example, joining two 10mm boards would generally call for a bolt of at least 20–25mm (about 13/16"–1"). When in doubt, size up — you can always trim excess thread, but a bolt that's too short won't grip securely.

A2 stainless steel (also known as 304 stainless) is an austenitic grade with a tensile strength of typically 500–700 MPa. It offers a solid balance of strength and corrosion resistance, making it suitable for most indoor and light outdoor fastening applications. For higher-load structural connections, consult an engineer.

Yes, A2 stainless steel provides good resistance to rust and oxidation in most outdoor environments. It performs well in rain, humidity, and mild coastal conditions. For highly aggressive marine or saltwater environments, A4 (316) stainless would generally be a better choice.

These bolts follow the ISO metric coarse thread standard (DIN 603 profile). M3 typically has a 0.5mm pitch, M4 a 0.7mm pitch, and M5 a 0.8mm pitch. The included hex flange nuts are matched to the same metric coarse thread, and in most cases they will also be compatible with standard ISO metric hex nuts of the same size.

A hex flange nut has a wide integrated washer-like flange at its base. This distributes clamping load over a larger surface area, reducing the chance of the nut pulling through softer materials like wood or plastic. It also eliminates the need for a separate washer on the nut side, simplifying assembly — a practical advantage for DIY woodworking and furniture projects.

In most cases, no. The hex flange nut's built-in flange acts as an integrated washer on the nut side. On the bolt head side, the domed carriage bolt head seats flush against the material surface. For very soft materials (e.g., thin plywood or MDF), adding a flat washer under the bolt head is generally recommended to prevent the head from sinking in.

Drill a hole matching the bolt diameter (e.g., 3mm hole for M3, 4mm for M4, 5mm for M5). Insert the bolt from the front face of the material and tap the domed head flush — the square neck will bite into the wood or material, preventing the bolt from spinning. Thread the hex flange nut from the back and tighten with a wrench. No tool is needed on the bolt head side, which is one of the key advantages of the carriage bolt design.