Cup Square Carriage Bolts with Hex Nut — Zinc Plated, M4 to M12

Generally, add the total thickness of all materials being joined, then add enough for the hex nut and at least one thread of engagement beyond it. For example, joining two 20mm (≈3/4\u2033) boards typically calls for a bolt around 50mm (≈2\u2033) long. When in doubt, size up — a slightly longer bolt is easier to trim than a short one is to replace.

These bolts use standard ISO metric coarse threads (M4, M5, M6, M8, M10, M12). They are compatible with standard metric hex nuts and metric-threaded holes of the same diameter.

The zinc plating provides moderate corrosion resistance suitable for covered outdoor structures and dry interior applications. For prolonged direct exposure to rain, salt air, or ground contact, stainless steel or hot-dip galvanized fasteners are generally recommended.

These are carbon steel bolts with zinc plating, typically meeting Grade 4.8 or equivalent tensile strength. For structural or load-bearing applications, consult your local building code and verify the grade marking on the bolt head before use.

A flat washer under the hex nut is generally recommended when fastening into soft materials like timber, as it distributes the clamping load and prevents the nut from pulling through. The dome head side does not require a washer — it seats flush against the surface.

The square neck just below the dome head is designed to bite into timber or soft material as the nut is tightened. This locks the bolt in place so you can tighten the nut with a single wrench — no need to hold the head. In harder materials like metal, a square hole or a second wrench may be needed.

Depending on the size selected, kits are available in quantities of 2, 5, 10, 20, or 50 pieces per pack. Smaller sizes (M4, M5) are available in larger packs of up to 50 pieces, while larger sizes (M10, M12) come in smaller packs of 2–5 pieces.