M8 M10 M12 Carriage Bolts, 304 Stainless Steel Round Head Square Neck Screws

Measure the total thickness of all materials being joined, then add typically 5–10mm for the nut and washer. For example, joining two 20mm boards generally calls for a bolt around 45–50mm (≈1-3/4" to 2") long. Always confirm the grip length before ordering. If you are between sizes, choose the longer option to ensure full thread engagement.

304 stainless steel carriage bolts generally have a tensile strength of approximately 500–700 MPa, depending on diameter and manufacturing tolerances. For heavy structural loads or safety-critical applications, consult a structural engineer to confirm suitability for your specific use case.

304 stainless steel offers good corrosion resistance for most outdoor environments including decks, fences, and pergolas. For coastal or marine applications with constant saltwater exposure, 316 stainless steel is typically recommended for superior chloride resistance. In most inland outdoor settings, 304 SS performs well over many years.

These bolts use standard metric coarse thread (ISO 68-1). M8 bolts are typically 1.25mm pitch, M10 bolts 1.5mm pitch, and M12 bolts 1.75mm pitch. They are compatible with standard metric hex nuts, flange nuts, and flat washers in the corresponding size — all readily available at hardware stores.

No special tools are required on the bolt head side. The square neck under the dome head locks into a pre-drilled square hole — or embeds into wood — preventing rotation while you tighten the nut from the opposite side with a standard wrench or socket. For wood applications, you can typically drive the bolt through a round hole and the square neck will self-embed as you tighten.

M8 (≈5/16" diameter) is generally suitable for light-to-medium structural connections such as furniture, light fencing, and shelving. M10 (≈3/8") is commonly used for decking, pergolas, and medium-load framing. M12 (≈1/2") is typically chosen for heavy structural applications like timber framing, retaining wall anchors, and playground equipment. When in doubt, size up for added safety margin.

In most cases, yes. However, the square neck anti-rotation feature works best in wood or soft materials where it can embed. For aluminum or hard composites, you may need to use a square hole punch or a backing plate to prevent the neck from spinning during installation. Contact us if you have a specific application question.