A carriage bolt, coach bolt or round head square neck bolt is a form of bolt used to fasten metal to wood.
It is distinguished from other bolts by its shallow mushroom head and that the shank cross-section of the bolt is circular for most of its length, as usual, but the portion immediately beneath the head is formed into a square section. This makes the bolt self-locking when placed through a square hole in a metal strap. This allows the fastener to be installed with only a single tool, a spanner or wrench, working from one side. The head of a carriage bolt is usually a shallow dome. The squared section is of the same size as the diameter of the bolt shank, with a plain unthreaded shank.
Carriage bolts were developed for use through iron strengthening plates on either side of a wooden beam, the squared section fitting into a square hole in the ironwork. It is commonplace though to use them to bare timber, the squared section giving enough grip to prevent rotation.
Technical Parameters:
| Diameter | 6 mm | Material | Galvanized steel | |
| Thread | Partial | Neck size | 6.48 mm | |
| Weight (100 pcs) | 0.76 ÷ 2.50 kg | Items per box | 800 |
