Coach Bolts vs Coach Screws

Coach bolts & coach screws sound like they are very similar, but they are very different types of fastener used for difference applications.

Here we look at the key differences between the two, and the different applications that they are used in.

What is a coach bolt?
A coach bolt, also otherwise known as a carriage bolt, is a machine threaded fastener and must be secured with either a matching nut or threaded insert, making it a 'two-component' fixing. Coach bolts are used for timber to timber applications and are especially useful when you have limited access to both sides of the bolt when tightening. This is because coach bolts have a square under the head which bites into the timber on the head side to stop it from turning, meaning you just need access to the nut side to fasten.

Coach bolts are typically made to din standard DIN 603 - you can view a size table here - and are mostly made from mild steel. We get many requests for high tensile coach bolts with the assumption that this means they are better, but in actual fact, coach bolts are deliberately made from mild steel because it means they can bend and accommodate movement in the timbers over time without snapping.

What is a coach screw?
A coach screw, also otherwise known as a lag screw, or confusingly, a lag bolt, has a coarse single wood thread that is designed to be fixed into timber, making it a 'single-component' fixing. A coach screw is normally fixed into timber, but you can also fix them into nylon wall plugs to make a heavy duty fixing into masonry. Coach screws do not come with nuts, nor do they need nuts, as the coarse single thread is designed to fix straight into timber. Coach screws are mostly used for timber to timber applications, but they can also be used for metal to timber, or timber to masonry applications.

Coach screws are typically made to DIN 571 and are mostly made from mild steel, for the same reasons pointed out for coach bolts above. Coach bolts are mostly partially threaded, although this is not specified in DIN 571 so it can vary. The thread length will always be at least 60% of the total length of the screw.