Carriage Bolt

A carriage bolt is a bolt with a round or mushroom head and a square neck under the head. The square neck bites into wood or metal to stop the bolt turning as the nut is tightened.

A carriage bolt passes through a drilled hole and its square neck bites into the timber or metal to stop it turning while a nut is tightened. It takes a nut and often a washer of the same thread, and it is driven with a hammer or mallet rather than a spanner. The bolt suits timber framing, fencing, decking and general assembly where a smooth round head is wanted.

Zinc plating gives mild corrosion protection for indoor or dry use. Hot-dip galvanising gives a thicker coating for outdoor and damp conditions. Plain finish has no coating and suits dry indoor work.

This kind of bolt is often made in steel, stainless steel 304 or 316, or brass. Stainless steel 316 suits wet or salty sites, and brass suits decorative work. Steel gives general strength at lower cost.

This is a general guide to checking this kind of part. It says nothing about any supplier's parts.

Ask for a certificate of conformance and a mill test certificate for the steel or stainless steel. On receipt, check the head shape, the square neck and the thread against your order. Check the finish for bare patches or uneven coating.