Cup Head Bolt

Product Description

A cup head bolt, commonly known as a carriage bolt or coach bolt, is a specialized fastener designed for use in timber-to-timber or timber-to-metal connections. Its most distinguishing feature is the smooth, domed head and a square section immediately beneath it.

Design and Anatomy

• The Head: A shallow, circular dome (the "cup") with a low profile. It lacks any drive socket (like a slot or hex hole), making it difficult to loosen or tamper with from the outside.

• The Square Neck: A square-shaped section located just under the head. When pulled into a pre-drilled hole in wood, this neck bites into the material, preventing the bolt from spinning as a nut is tightened on the opposite end.

• The Shank: Typically features a standard machine thread (metric or imperial) that extends down to the tip.

Why Use a Cup Head Bolt?

• Security: Since the head is smooth and cannot be gripped by a wrench or screwdriver, these bolts are popular for outdoor gates, fences, and security doors where you want to prevent unauthorized removal.

• Safety and Aesthetics: The low-profile, rounded head reduces the risk of snagging clothes or skin, making it the standard choice for playground equipment, park benches, and decking.

• Self-Locking: The square neck allows for one-sided installation. Once the bolt is seated, you only need one wrench to tighten the nut from the back.

Common Specifications

Installation Tips

• Hole Sizing: The hole should be drilled to match the diameter of the threaded shank. Do not drill a square hole; the square neck is designed to "self-seat" into the round hole as the nut is tightened.

• Washers: It is highly recommended to use a large flat washer (fender washer) under the nut. This prevents the nut from sinking into the wood and ensures a more secure, distributed clamping force.

• Hardwood vs. Softwood: In very hard timbers, you may need to slightly countersink the square neck area to avoid splitting the wood, though in softwoods like pine, the bolt will pull itself in easily.