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I have a question about carriage bolts, and when they are appropriate to use, instead of more traditional hex head bolts. If I understand correctly, their main advantage is that they can be tightened from only one direction, the square part under the bolt head providing the resistance. With a hex bolt on the other hand, you have to use a wrench to hold the head in place while tightening (or untightening). Is this the main advantage (aesthetics aside)?

Are the carriage bolts square part able to provide that resistance in soft wood such as pine or cedar? Or should they be used mostly in hard wood? Could the square part tear through soft wood and start to spin while being tightened?

For reference, the picture below shows what I mean when I say carriage bolts vs hex head bolts. I've seen some terminology confusion on the internet.

enter image description here

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    While the current crop of answers are fairly spot on, the other advantage of carriage bolts is that they are more secured against theft when used on the outside of door hardware. With a standard hex-head bolt, someone could put a wrench on the outside and has a chance of loosening the bolt & removing whatever it was holding. With the carriage bolt, there's nothing to get a wrench on from the outside and the only option would be to attempt to grind the bolt which requires power tools, time and noise, alerting others to the activity.
    – FreeMan
    Jan 18 at 12:28

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A carriage bolt head has several advantages relating to wood.

First and foremost, it's a smoother surface. That head is fairly low-profile and ramped. That makes it less likely to snag things or people.

When this is done to extremes, it is an elevator bolt, with a completely flat and slightly tapered head, designed to be pulled down to completely flush in soft wood.

enter image description here

The head on a typical carriage bolt is also much wider than a hex head - this to spread the load on compressible wood. Your photo's hex bolt is oddly truncated, like someone turned it down on a lathe. That one might be intended for metal assembly.

Generally speaking, in healthy wood, "cam-out" of the square isn't a big problem. Keep in mind with softwood, torque is limited anyway due to the risk of crushing the wood. What gets carriage bolts (maddeningly) is when the wood gets weather-damaged and the bolt gets rusty - then the breakaway torque on the nut vastly exceeds the cam-out of the square.

Note that with metal, there's a generous chamfer/radius between the square cross-section and the bottom of the bolt head. If you broach a square hole in steel that is tight to the square dimension, the carriage bolt will sit proud of the steel surface, with all the force concentrated on the edge of the hole at the chamfer/radius. You have to make the dimensions a bit sloppy for that to work. In thicker plate there's not really a point, as it's easier to drill undersize and tap a screw thread right into the metal, so you use a bolt on the other side instead of a nut. Also think about how many hobbyists own taps, vs how many own broaches.

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    While in general I agree with what you've said, I have seen many usages of carriage bolts in metal, where loads are low, such as outdoor gas grills, shelving units, etc, to fasten sheet metal parts together. The square holes aren't much trouble to fabricate when it's in stamped sheets made hundreds to thousands of units at a time. Jan 18 at 4:51
  • @whatsisname OK, I toned that down. Jan 18 at 6:30
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    Rusty tee nuts and spring washers in rotten wood are no more fun than rusty carriage bolts. Ask anyone who's disassembled an old playground which they prefer and they'll tell you .... they prefer a sledgehammer and a sawzall. ... Never heard of an elevator bolt before, I'm going to try to use that in a project!
    – jay613
    Jan 18 at 11:44
  • @jay613 they're a bit scarce. We needed ~400 quantity for a project in a longer length e.g. 4", our purchasing agent couldn't find them. We wound up making extension sleeves by turning down hex couplers on a lathe... 200 of them (we only did half). Jan 18 at 18:42
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If the hole is the proper size, carriage bolts work fine in wood, both soft and hard. You'll generally crush the wood via over-tightening before you'll spin the bolt head in the wood.

A friend teaches bookmaking and makes budget book clamps from carriage bolts and ordinary SPF 2x4s. I've never seen one of those spin out. They get cranked on pretty hard.

Now, if the wood gets rotten, the threads get rusty, and you try to remove it - yes, then you'll spin the head, and end up cutting it off.

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Carriage bolts can be used in sheet metal with punched square holes, like for shelving or in wood.

For similar functionality (one-sided tightening) in wood you could use tee nuts
enter image description here

or spring washers
enter image description here

or various furniture fasteners that work similarly but are embedded inside the wood for a better look.

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    No, that's not right. Lots of woodwork uses carriage bolts. Like, you know, carriages :) I have broached square holes in metal to use them, but it's a PITA. Jan 18 at 0:51
  • T-nuts are generally used where the fastener may have to be removed and reinserted over the lifetime of the item. Carriage bolts are meant to stay in place forever. Jan 18 at 4:52
  • Modified the answer a bit, thanks @Harper-ReinstateMonica. Saw some good youtube tutorials now on using carriage bolts in wood.
    – jay613
    Jan 18 at 11:34

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