A lot of people are asking me these days, “Hey, David, what’s the structural difference between a machine bolt and a carriage bolt?”
That’s a lie. Nobody actually asks me the difference. I tell people the difference because I think it’s important. But then again, I’m a nerd about these things. It all comes down to the head. Carriage bolts have a rounded head on top that shrinks to a square underneath before connecting to the shaft of the bolt. This funky shape allows the carriage bolt to be good for one specific thing. Okay, maybe two specific things. The round head will not catch, snag, scrape, or otherwise entangle any passersby. Think of it like that goth kid who sits at the back of the class and won’t keep his hood down no matter how many times the teacher tells him. That kid will totally avoid interacting with anyone as if his life depended on it.
Um, let’s step away from that overly complicated and saddening analogy. The carriage bolt has an ace up its sleeve in the form of that little square knob underneath the head. When the bolt is tightened against something softer than metal (playset wood, for instance), the square knob will bite into the surface and provide extra grip for the whole bolt. It’s a great system…sometimes.
The catch is this: as soon as the wood begins to soften (as it always will over time) the bolt head will have nothing to hold onto. Because playsets are always getting jumped on, swung on, slid on, climbed on, etc, there’s a lot of strain put on a set’s joints where the main structure parts come together. That’s where the carriage bolts are. So just re-tighten the bolts, right? Problem solved!
Yeah, no. That little square head needs something to hold in order to pull the bolt tight. Without anything to grab, it’ll just spin in place. This makes re-tightening a loose carriage bolt difficult at best and impossible at worst.
Playsets everywhere going wobbly at the joints; becoming unsafe for our children to play on! Who shall save us from this calamity? Never fear, the Mighty Machine Bolt stands ready to save us!
Wow, that was ridiculous. I apologize. Let’s talk details. The machine bolt sacrifices the carriage bolt’s smooth rounded head for a clunky multi-sided one. What it gains is the ability to be tightened over and over again. This means that no matter how hard kids play on a playset whose joints are held by machine bolts, the set can always be made as firm and sound as new with nothing more than a simple socket set and a can-do attitude!
But what about that whole “won’t snag, won’t scrape” thing with the carriage bolts? A machine bolt will snaggle anyone who brushes by it like that bubbly kid in school who always becomes your best friend as soon as she meets you, then won’t leave you alone in the cafeteria, the library, anywhere!
I’m getting off-track again. Let me introduce you to the concept of countersinking. Countersinking a bolt hole basically means that a larger, shallower hole is drilled in the same surface around the small bolt hole. The larger countersink is just big enough to fit a bolt head. Think of it as a bolt head garage, keeping your neighbor’s eyesore of a yellow hummer off his driveway where it was giving you a headache. Ta-da! No more bolt head to snag, scrape, etc.
That was the long way to say that playsets which use machine bolts with countersunk holes will always give you more mileage than ones with simple carriage bolts. When you go shopping for your kids’ next backyard imagination castle, keep this in mind. That’s all. Thanks for asking!