Black Carriage Bolts M3 M4 M5 M6 — Cup Square Domed Head, A2-70 Stainless Steel

As 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 bolt diameter. For example, joining two 10mm boards with an M5 bolt would generally require at least 25mm (≈1"): 20mm of material plus ~5mm for the nut. Always measure your stack-up before ordering. Quick inch reference: 25mm ≈ 1", 50mm ≈ 2", 80mm ≈ 3-1/8".

These bolts are A2-70 stainless steel (equivalent to 304 grade), with a minimum tensile strength of 700 MPa. Actual load capacity depends on the bolt diameter, length, and application — an M6 bolt will generally carry significantly more load than an M3. For safety-critical structural applications, consult an engineer and verify load ratings against your specific design requirements.

A2 stainless steel offers good corrosion resistance and is generally suitable for outdoor use in most climates. For highly aggressive environments — prolonged saltwater exposure, coastal spray zones — A4 (316) stainless is typically recommended. The black surface finish may show wear over time in harsh conditions, though the underlying stainless steel will remain corrosion-resistant.

Yes. These bolts use standard ISO metric coarse threads (DIN 603 specification) in M3, M4, M5, and M6. They are compatible with any standard metric hex nut, flange nut, or wing nut of the matching size. They are not compatible with UNC or UNF imperial threaded hardware.

A cup square (carriage bolt) head has a smooth rounded dome on top with no drive recess — it's designed to sit flush and clean on the visible face. Directly below the head is a short square shank section. When inserted into a pre-drilled round hole in wood, the square section bites into the material as the nut is tightened, locking the bolt against rotation. This means you can tighten the nut with one hand and one tool, with no need to hold the bolt head.

Yes — these carriage bolts work in wood-to-wood, wood-to-metal, and metal-to-metal assemblies. The square neck is most effective in wood or softer materials where it can embed and resist rotation. For metal-to-metal use, a square punched hole or an anti-rotation plate is generally needed to prevent spinning. They are also suitable for use with plastic panels when the hole is sized appropriately.

Quick reference conversions for the sizes in this listing: 8mm ≈ 5/16", 10mm ≈ 3/8", 12mm ≈ 15/32", 16mm ≈ 5/8", 20mm ≈ 25/32", 25mm ≈ 1", 30mm ≈ 1-3/16", 35mm ≈ 1-3/8", 40mm ≈ 1-9/16", 45mm ≈ 1-3/4", 50mm ≈ 2", 60mm ≈ 2-3/8", 70mm ≈ 2-3/4", 80mm ≈ 3-1/8".

Thread diameter selection generally depends on the load and the material thickness. M3 is typically used for light-duty applications like electronics enclosures or thin sheet assemblies. M4 suits small furniture and light brackets. M5 is a common general-purpose size for furniture, shelving, and light structural work. M6 is appropriate for heavier structural timber connections, outdoor furniture, and applications requiring higher clamping force. When in doubt, size up — a larger bolt in a slightly oversized hole is easier to work with than an undersized bolt under load.