What makes a Carriage Bolt a Carriage Bolt?

What is a carriage bolt and what is it used for?

Carriage bolts, also known as coach bolts or cup head bolts in Australia and New Zealand, have a round head, a square neck, and are generally partially threaded. They were originally designed for fastening wood to metal, or metal to metal as the square neck area of the bolt was meant to fit in to a square socket in the metal. This portion of the neck serves to hold the bolt stationary as a nut is screwed on or off. These days, we use carriage bolts for all kinds of uses, and when fastening on a boat they are most often used for fastening wood to wood, but could also be used for fastening wood to metal such as in bronze knees.

Why are carriage bolts only partially threaded?

The unthreaded portion of a carriage bolt is there for a reason, I assure you! Structures, and boats in particular, work, flex, twist, and move when they're put to use. Even the strongest, stoutest built vessel is alive with movement when subjected to all of the forces put on it out in the water. This movement is exactly why having a partially threaded bolt is important.

Carriage Bolt in WoodTo the left here is a simple diagram of a carriage bolt in wood. Imagine the grey area is a piece of timber, or two pieces of timber joined on a vessel's structure. As the boat flexes while pounding in to a wake or wave that wood will ever so slightly move. In the diagram the wood comes in to contact with the unthreaded portion of the bolt's shank, and as the wood moves the bolt won't bore out it's clearance hole, and won't saw those pieces of timber apart. If that bolt was fully threaded the threads would saw on the inside of that hole, reaming it out and making the fitment of those two pieces of timber worse as time goes on.

Know what kind of carriage bolt you're buying!

I recently checked out some of my competitors' products and was surprised at how inexpensive some of their carriage bolts are. Having gone through lots of product testing myself I was surprised they could sell a carriage bolt at such a price. Then I ordered some...

Silicon Bronze Carriage Bolt Comparison
The image above shows what I received. Both of these Silicon Bronze "Carriage Bolts" are 1/4"-20 x 3", but the one on top is what I received after purchasing from one of our competitors and the one below is what I stock and sell here at Fair Wind Fasteners.
As you can see, the top bolt is fully threaded, and if it were used in a boat applicating it would gradually saw open it's clearance hole and degrade the timbers it was inserted in to.
You'll also notice that the shank and the head on the lower bolt are both much thicker and beefier. That is because the top bolt is threaded through a cheaper and faster manufacturing method known as rolling the threads, whereas the bolt on the bottom has cut threads which allows for a thicker shank. You can learn more about cut vs. rolled threads here.
Furthermore, the retailer that I purchased the top carriage bolt from just says they are "Silicon Bronze" and there is a whole multitude of "Silicon Bronze" alloys that it could be. We guarantee our carriage bolts to be true C65500 High Silicon Bronze "A". I'm not sure what that other one is made of...

Quality Matters!!!

It's so easy to purchase low grade fasteners (especially online) but is that what you really want to use? When pouring time, effort, blood, sweat, and tears in to a boat build or restoration it would be disheartening to see it all come undone by fasteners that are not up to the task because they're the wrong alloy or in this case are threaded where they shouldn't be. Trust that at Fair Wind Fasteners we've taken the time to test for quality, and would NEVER sell anything we wouldn't use ourselves.

Have a look at some of the comparison photos below, you'll easily be able to see which Silicon Bronze carriage bolt is ours, and which is theirs. Is it even a carriage bolt if it's fully threaded? I say no.

Silicon Bronze Carriage Bolt
Bad Carriage Bolts