M4 M5 M6 Carriage Bolts – A2 Stainless Steel Cup Square Dome Head Screws

A general rule: the bolt length should equal the combined thickness of all materials being joined, plus enough thread engagement for the nut — typically at least 1× the thread diameter beyond the last material. For example, joining two 10mm (≈3/8") boards with M6 bolts, a 25mm (≈1") bolt is generally sufficient. Always measure your stack-up before ordering.

A2 stainless steel to ISO 3506 property class A2-70 typically has a minimum tensile strength of 700 MPa (≈101,500 psi). This is generally adequate for furniture, decking, and general structural joinery. For safety-critical or high-load applications, consult a structural engineer.

Yes. A2 stainless steel resists rust and oxidation well in most outdoor conditions including rain, humidity, and garden environments. For coastal or marine locations with heavy salt spray, A4 (316) stainless steel is generally the better choice for long-term corrosion resistance.

These bolts conform to DIN 603 / ISO 8677 metric coarse thread. M4 has a 0.7mm pitch, M5 has a 0.8mm pitch, and M6 has a 1.0mm pitch. They are compatible with any standard metric hex nut, nylon-insert lock nut, or wing nut of the matching size available at hardware stores.

A flat washer under the nut is generally recommended to spread the clamping load and protect the surface. The dome head on the bolt side sits flush and does not typically require a washer, though one can be added when fastening softer or more compressible materials like plywood or composite decking.

The square section directly beneath the dome head is designed to embed into wood or soft composite material as the nut is tightened. This mechanical lock holds the bolt stationary, so you can drive the nut with one hand without needing to grip the head — a key advantage of the carriage bolt design over standard hex bolts.

For small repairs or prototyping, packs of 5–10 pieces are typically cost-effective. For furniture builds or deck projects, packs of 50–100 offer better per-unit value. Bulk packs of 200–500 are suited for contractors or production runs where a consistent metric fastener size is needed across many joints.