Cup Square Bolts

A cup square bolt, also called a carriage bolt or coach bolt, has a rounded head and a square shoulder underneath that pulls into timber and stops the bolt turning while you tighten the nut. Simfix stocks them to DIN 603 in A2 (304) and A4 (316), from M5 to M16 and from 12mm to 300mm long, depending on diameter.

Used for fencing, decking, gates, pergolas and post supports. Choose A2 for inland outdoor work and A4 near the coast or where timber stays wet.

Frequently asked questions

What is a cup square bolt?

A cup square bolt has a rounded (cup) head and a square section directly under it. The square shoulder pulls into timber as you tighten the nut, stopping the bolt from spinning. It is also called a carriage bolt or coach bolt. Simfix stocks cup square bolts to DIN 603 in A2 (304) and A4 (316), M5 to M16.

Carriage bolt vs coach bolt vs coach screw: what's the difference?

Carriage bolt and coach bolt are two UK names for the same fastener, called a cup square bolt at Simfix (DIN 603): a bolt with a rounded head, tightened with a nut. A coach screw has a hex head and a wood screw thread instead of a nut, and is driven with a spanner into a pre-drilled hole, to DIN 571.

Are cup square bolts available in A4 (316)?

Yes. Simfix stocks DIN 603 cup square bolts in A4 (316) from M5 to M16, as well as in A2 (304). Choose A4 for coastal or salt-exposed timber and for timber that stays wet; A2 suits fencing, decking and gates away from the coast.

What sizes of cup square bolt does Simfix stock?

M5, M6, M8, M10, M12 and M16 diameters in both A2 (304) and A4 (316), in lengths from 12mm to 300mm depending on diameter. Each product listing is a single diameter and length, and not every length is stocked at every size and grade.

Sizes, grades and pack quantities vary by product type. Check the individual product listing for the exact specification and current availability.