The Difference Between Bolts and Screws
What is the difference between a bolt and a screw?
It seems that the terms are often used interchangeably, but there is indeed a technical difference between bolts and screws. We'll put an end to the debate you're having with the lead boatbuilder or foreman on the job right now. The bolts vs. screws chat ends here! So what is it? What makes a bolt a bolt and a screw a screw?
Size Doesn't Matter!
When your girlfriend told you that she was probably lying, but I can assure you that in this case I am not. The size of the fastener has nothing to do with it being a bolt or a screw. Often times we'll look at a large fastener and want to call it a bolt, and likewise hold a smaller fastener and want to call it a screw. When it comes to classifying a fastener as either a screw or a bolt size has nothing to do with the matter (it's not the motion of the ocean either). This big bronze fastener below? Yeah, that's a screw!
The Drive Type Doesn't Matter!
A lot of people think that if you have to use a wrench to drive it in then it's a bolt, and if you use a screwdriver to drive it in then it is a screw. That would make everything with a hex head a bolt, and everything with a slotted, frearson, square, torx, or some other kind of drive a screw. However, that's not correct either. That big, gorgeous 651 silicon bronze fastener to the right? That's a screw as well.
Set aside the fact that there is a hex head on that fastener and the fact that many people would refer to it as a bolt, it is still technically a screw. Both examples above and to the right are screws, do you see any similarities? Of course, it's the threads!
The thread type is what makes a screw a screw and a bolt a bolt.
The difference between a bolt and a screw has nothing to do with whether or not a wrench is used to drive it in, or how big it might be, but everything to do with the type of threads on the fastener.
On the left here is what most people would casually refer to as a lag bolt, and a machine screw. I know I often have. However, they are technically lag screws and machine bolts due to their thread pattern. Quite simply, the threads of a screw mate with the material it is turned through to make it's own threads while a bolt is meant to be used with a pre-threaded hole or fastened with a nut, (in this case a bronze nut for the bronze bolts to prevent any galvanic corrosion).
But that leaves so many questions!
It does. But we have answers:
1. So a machine screw is actually a bolt?
Absolutely. "Machine Screws" are technically machine bolts. Machine bolts fasten by mating with a nut or threaded hole which does indeed make them bolts. Common parlance is to call them machine screws, and I'll admit that I always have, but that is technically incorrect. We even have ours listed in the bolts section of our website.
2. Then a lag bolt is actually a screw?
Yes again. "Lag Bolts" are also technically screws because you can't thread a nut on to them and they do make their own threads when driven in to a piece of wood or other material. Forget about the fact that you need a wrench to drive them in and they have a hex head on them, they are technically lag screws. Of course most people do refer to these fasteners as lag bolts, so for ease of use and navigation we've included them in the bolts section of our site.
3. What about a bolt with a "thread cutting" tip?
Some "bolts" are able to cut their own threads due to a thread cutting pattern on the tip of the fastener that will cut the material it is driven in to in order to help the fastener form threads in that material. Fasteners like that are ALSO technically screws since they are not meant to be used with a nut or pre-threaded hole.
4. Which is stronger, a bolt or a screw?
With all other things being equal, and remembering that the size of the fastener does not define it as a bolt or a screw, a bolt does provide a stronger fastening method and greater holding strength. Why? Bolts are designed to use a nut and washer, and that washer will help distribute the load over a greater surface area.
5. What are the advantages of using a bolt vs a screw?
As we just learned, a bolt does have greater holding strength so if strength is the concern then a bolt is superior in that regard. Another advantage is the ease of removal later on down the line. Screw threads will work and damage wood over time and that wood won't hold up well to several re-installations of a screw, but a bolt can be used over and over with no ill effects. When using our bronze bolts with bronze nuts and washers you won't have to worry about rust fusing together your fasteners making them frustratingly difficult to get apart, and you can have even greater confidence of removal years later by dabbing on a little bit of thread lubricant to protect the threads from seizing and galling.
6. What are the advantages of using a screw instead of a bolt?
Despite bolts being stronger, screws do have their place and do have some advantages. Sometimes it's just not practical to access both sides of a fastener, and a screw can be driven in from just one side of your project. In that case, a screw is not only more practical but the only way to go! You might also not want to drill a large clearance hole in your project to make way for a bolt, in which case a screw would work better. When boatbuilding, screws are used for planking because they can be driven snugly in to a smaller pilot hole and create a waterproof seal around the fastener which would be hard to achieve with a bolt. Just make sure you're using a screw with a full bodied shank and cut threads like the ones we have here at Fair Wind Fasteners.
Hopefully we've cleared that up!
Bolts require nuts or a threaded hole for them to fasten your project, and screws make their own threads (and you'd look ridiculous if you tried to put a bolt on them). However, we know that most people still want to call a lag screw a lag bolt, and a machine bolt a machine screw. That's okay - we get it! No matter what you call them, we are pleased to provide all of your high quality, specialty silicon bronze fasteners here at Fair Wind Fasteners.