Coach Bolts / Carriage Bolts / Cup Square Bolts

Coach Bolts – A Tradefix Direct Guide

Coach bolts (aka carriage bolts or cup square bolts) are a staple construction fixing, ideal for timber to timber applications, and for attaching ironmongery to wood.

When should I use coach bolts?

Coach bolts are the natural choice for most large building projects and can be suitable for interior and exterior applications. They are typically used at critical points within a structure to join wood to wood or wood to metal. They offer a far more secure option than nails, screws and many other bolt types.

Coach bolt sizes

You’ll see the product descriptions of our coach bolts have two dimensions: size and length. The length is obvious enough, but what does M8, M12 etc mean? M is simply the metric diameter of the bolt, so an M10 bolt has a thread diameter of 10mm, an M16 bolt is 16mm wide. Note that the M measurement applies to the thread, not the head of the bolt, which is always wider.

Parts of a carriage bolt

Head: That broad head doesn’t just look good. It’s there to spread the surface load of the carriage bolt so it doesn’t damage the wood or other material you’re bolting. For extra protection, consider choosing a washer.

Square section: Beneath the domed head, at the top of the shank is a short square block. It is designed to be countersunk into the timber and ensures that when the nut is tightened the bolt grips the timber and doesn’t simply spin. This part is what gives them yet another name: cup square bolts.

Shank: Take a close look at our Timco coach bolts and you’ll see a portion of the shaft is unthreaded. The same is true for some of our unbranded carriage bolts too. Why is that? A shank helps the bolt pull together the materials it is fixing. It also helps reduce friction and should make it easier to remove the bolt.

Which finish do I need?

Our coach bolts are available in a range of materials/finishes. Zinc plated coach bolts are typically the lowest cost option. Galvanised coach bolts are also, rather confusingly, zinc coated but the coating is thicker and offers greater protection. Stainless steel coach bolts are the most durable of all, because the corrosion resistance comes not from the coating but from the composition of the bolt (which combines steel with chromium).

Indoors and where the environment is dry and not subject to humidity, any of the bolts should be suitable. If you need carriage bolts for exterior applications, or in damp or humid environments, galvanised bolts will offer greater corrosion resistance than zinc coated, whilst stainless steel would be the most durable option of all.

Buy coach and carriage bolts at Tradefix Direct

No other UK coach bolts supplier offers a better combination of range, quality and value than Tradefix Direct. Order your cup square bolts now and receive free next day delivery across the UK on orders over £60. Great price break discounts on large orders for a wide range of selected items are also available. If you’re not sure which is the right carriage bolt for your application, call us on 0161 794 1783. We’ll be happy to help!