M3 M4 M5 Stainless Steel Carriage Bolts — DIN 603 Cup Square Dome Head, 304 SS

Generally, the bolt length should equal the total thickness of the materials being joined plus enough thread engagement for the nut — typically at least 1× the thread diameter beyond the material. For example, joining two 10mm boards with M5 bolts would typically require at least 25mm (≈1") length. Always factor in a washer and nut when calculating the required thread protrusion.

304 stainless steel carriage bolts typically have a tensile strength of around 500–700 MPa. Actual load capacity depends on thread diameter, length, and the materials being fastened. For structural or load-critical applications, consult an engineer and verify the specific grade requirements for your project.

304 stainless steel offers good corrosion resistance and is generally suitable for outdoor use in most environments, including decks, fences, and pergolas. For highly corrosive or marine environments with constant saltwater exposure, 316 stainless steel is generally recommended. These bolts perform well in typical outdoor DIY and construction applications.

These bolts conform to DIN 603 and use standard metric coarse threads (M3, M4, or M5). They are compatible with standard metric hex nuts and flat washers of the corresponding size, which are widely available at hardware stores across the US.

Carriage bolts are designed to be tightened from the nut side only. The square neck under the dome head bites into wood or soft material to prevent the bolt from spinning. You only need a wrench or socket on the nut — no screwdriver or hex key is required on the bolt head. This makes them especially convenient for one-person installation.

Thread diameter selection depends on the load and material thickness. M3 (≈0.12" diameter) is typically used for light-duty or thin-material applications. M4 (≈0.16") suits medium-duty joints and general woodworking. M5 (≈0.20") is appropriate for heavier loads or thicker timber stock. When in doubt, size up for an added safety margin.

Carriage bolts are primarily designed for wood-to-wood or wood-to-metal connections, where the square neck locks into the material to prevent spinning. In metal-to-metal applications, the square neck cannot grip, so the bolt may rotate during tightening. For metal-to-metal use, a hex bolt or socket head cap screw is generally more appropriate.