When is a screw not a screw: An examination of fastener nomenclature.

October 24, 2013 11:06 PM

The difference between a bolt and a screw is a controversial topic. Confusingly, even some screws can also be bolts. Thankfully, the department of homeland security is on the case. The DHS notes, perhaps predictably, that "international standards are not necessarily applicable" to the US. In conclusion, fasteners are a land of contrasts.
posted by empath (90 comments total)

I approve of this thread.
posted by chemoboy at 11:07 PM on October 24, 2013 [30 favorites]

Yes, it will surely be riveting.
posted by 7segment at 11:11 PM on October 24, 2013 [18 favorites]

Dammit, the good puns are all taken. Tapped out as it were.
posted by ish__ at 11:11 PM on October 24, 2013 [6 favorites]

You guys are nuts.
posted by empath at 11:11 PM on October 24, 2013 [8 favorites]

I actually agree with the DHS, it's rather simple. A bolt is used in conjunction with a nut or other threaded element to fasten, whereas a screw uses its threads to fasten itself in the material. The 'lag bolt' should be called a lag screw. The drive (type of head) has no bearing on the terminology.
posted by Teakettle at 11:13 PM on October 24, 2013 [7 favorites]

Everyone just bolted in here to get this started...
posted by Tell Me No Lies at 11:14 PM on October 24, 2013 [1 favorite]

Fastenating.
posted by pracowity at 11:14 PM on October 24, 2013 [6 favorites]

(This comment intentionally left blank)
posted by pracowity at 11:14 PM on October 24, 2013 [1 favorite]

Tell Me No Lies: to be fair, several of us were talking about it on mefi chat for a good 15 minutes before this was posted..... Just waiting to jump in with our puns.
posted by ish__ at 11:15 PM on October 24, 2013

A bolt is used in conjunction with a nut or other threaded element to fasten, whereas a screw uses its threads to fasten itself in the material.

I'm going to preface this by saying it's been a slow night a work.

I thought the same thing, but what happens if you put a nut on a screw? Does it then become a bolt? My argument was that it is then a screw which has been used as a bolt, but my co-worker disagrees and thinks the terms are inherently ambiguous.
posted by empath at 11:15 PM on October 24, 2013

Teakettle, you are correct, but every rule has an exception. In machinery, the non tapered, hex headed screws that don't receive a nut (say as in a block fastener in a car engine) are referred to as bolts.

But yeah, that's a good rule of thumb.
posted by Xoebe at 11:15 PM on October 24, 2013 [1 favorite]

The distinction just hit me like a bolt out of the Blue.
posted by islander at 11:27 PM on October 24, 2013 [1 favorite]

Whoever has to apply those crazy rules all day is screwed.
posted by jewzilla at 11:32 PM on October 24, 2013 [2 favorites]

> A bolt is used in conjunction with a nut or other threaded element to fasten, whereas a screw uses its threads to fasten itself in the material.

I'm going to be less charitable than Xoebe and say that this is totally wrong. What, do you "screw on" a cylinder head to an engine block? Do you need to grab a socket wrench tighten the "screws" that hold one half of your garbage disposal to the other? No way, man. Them's bolts, lack of nuts notwithstanding.
posted by thedaniel at 11:33 PM on October 24, 2013 [2 favorites]

This entire discussion has been stripped of meaning. I suggest rethreading.
posted by trip and a half at 11:40 PM on October 24, 2013 [7 favorites]

If you rotate the fastener to drive it, it is a screw. If you rotate something around the fastener, it is a bolt. Of course Einstein might point out that this distinction is meaningless because the only thing that matters is that they're rotating relative to each other. Thus, bolt = screw.
posted by qxntpqbbbqxl at 11:47 PM on October 24, 2013 [5 favorites]

We obviously need to establish a distinction that's conserved under rotating reference frames, and I have no idea why that wasn't immediately obvious to the standards people.
posted by empath at 11:57 PM on October 24, 2013 [4 favorites]

Without checking all the links, I offer the following definition:

First day of the semester, the shop teacher walks in and asks the class "does anyone know the difference between a nail, a screw, and a bolt?"

No-one puts their hand up.

"C'mon!" he says, "someone must know the difference!"

One girl timidly puts her hand up.

"Finally! Kathy, tell the rest of the class the difference between a nail, a screw, and a bolt".

"Well" she says, quietly. "I've never been bolted …"

posted by Pinback at 12:16 AM on October 25, 2013 [13 favorites]

Ya gotta be nuts to try and bolt from the DHS - they can screw you right up.
posted by islander at 12:20 AM on October 25, 2013

2. DEFINITIONS

2.1 Bolt

A bolt is an externally threaded fastener designed for insertion through the holes in assembled parts, and is normally intended to be tightened or released by torquing a nut.

2.2 Screw

A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head.

2.3 Bloody Obvious

See 2.1 and 2.2.

Good thing the government shutdown ended.
posted by three blind mice at 12:21 AM on October 25, 2013

I just read a 21 page Department of Homeland Security document about the difference between bolts and screws.
posted by ActingTheGoat at 12:34 AM on October 25, 2013 [4 favorites]

Bolt before screwed.
posted by y2karl at 1:02 AM on October 25, 2013 [1 favorite]

But there are captive nuts,where you fasten by screwing in the bolt, And there are threaded inserts, ditto.

It's hard to gauge.
posted by Devonian at 1:16 AM on October 25, 2013 [1 favorite]

I'm galvanized...tell me more.
posted by tula at 1:17 AM on October 25, 2013 [3 favorites]

I thought the same thing, but what happens if you put a nut on a screw?

You ruin the screw.

Re: the exhaust manifold—it's a bolt, because it's going into a prethreaded hole. Also, cylindrical. Screws are tapered.
posted by five fresh fish at 1:22 AM on October 25, 2013 [2 favorites]

I thought the same thing, but what happens if you put a nut on a screw? Does it then become a bolt?

If you hammer a screw into some wood, does it become a nail?
posted by EndsOfInvention at 1:24 AM on October 25, 2013 [2 favorites]

A bolt is used in conjunction with a nut or other threaded element to fasten, whereas a screw uses its threads to fasten itself in the material.

So machine screws, like for example the ones that hold your light switch cover to the wall, or fasten your hard drive to your computer, are bolts?
posted by Pruitt-Igoe at 1:42 AM on October 25, 2013

What's a screw?

$5, same as in town.
posted by Segundus at 1:42 AM on October 25, 2013 [3 favorites]

So machine screws, like for example the ones that hold your light switch cover to the wall, or fasten your hard drive to your computer, are bolts?

Although I'd call them screws due to popular convention, I think of them as bolts because of their parallel sides. Real screws are tapered.
posted by PeterMcDermott at 1:54 AM on October 25, 2013 [2 favorites]

At the risk of derailing, I feel I must ask:

- if you take away the saucer, does a cup become a mug?
- if I tie up a parcel with thread, is it string?
- is a pillow on my sofa really a cushion, or is a cushion on the bed a pillow?

What are the distinguishing characteristics of a dish of beans, as compared with a plate?
posted by Segundus at 1:57 AM on October 25, 2013 [6 favorites]

There's a tendency for engineers to call something a screw where a layperson would say 'bolt'.

In the end, if you're ever in any doubt, it's ok to ask the threaded fastener politely what it prefers to be called. Haven't you people learned anything yet???
posted by pipeski at 2:24 AM on October 25, 2013 [3 favorites]

It seems like it would be a lot easier for everyone if the tariff schedules on screws and bolts were the same (I mean, surely if you can make one you can make the other, right?), so CBP wouldn't have to care at all. Looking up the tariff classifications, we find pages of different classifications for no particularly good reason. We also learn that safety pins and dressmakes' pins have slightly different rates of duty, unless they come from a list of certain countries, at which point they are free. This is a great use of government resources, not to mention my time right now...
posted by zachlipton at 2:40 AM on October 25, 2013 [1 favorite]

If it screws into pre-existing thread, I call it a bolt.
If it screws into non-threaded material, I call it a screw.

Furthermore, I second the notion that screws are tapered.
posted by Too-Ticky at 3:01 AM on October 25, 2013 [3 favorites]

I don't know about nomenclature, but what I do know is that I really really like the self drilling, self tapping screws I'm currently using.

Self Drilling! Self Tapping! Into Steel!

(The awesomeness of this may only be apparent if you've tried to drill and tap several hundred steel screw holes manually last month)
posted by Just this guy, y'know at 3:08 AM on October 25, 2013 [2 favorites]

Machine screws are not tapered, and go into a pre-existing thread. Same with set screws and many socket screws. Lag bolts are tapered, and do not need to go into a pre-existing thread.

In the nomenclature people actually use, the type of top really does seem to be the difference. Screws have a slotted top of some kind. Bolts do not. I guess you can call a tapered bolt a "lag" if that really bothers you for ... some reason.

(But kyrademon, hex head sheet metal screws don't have a slotted top, and -- ARG! OK! I'll forget hex head sheet metal screws exist if you just put down the knife! AIEEEE!)

*Cough* ... There may, of course, be a couple of exceptions to this rule.
posted by kyrademon at 3:17 AM on October 25, 2013 [2 favorites]

In the nomenclature people actually use, the type of top really does seem to be the difference. Screws have a slotted top of some kind. Bolts do not.

I have a couple of boxes of fasteners that would disagree with you. The defining characteristic of a bolt is that it does indeed take a nut. Screws don't. Head's got nothing to do with it, necessarily.

The Engineer Explains puts it well: "The words "bolt" and "screw" are ambiguous. Bolt or screw is not a physical thing; it is a matter of how a physical thing is used."
posted by middleclasstool at 3:42 AM on October 25, 2013 [1 favorite]

Is this a major issue? Is the smuggling of one disguised as the other some sort of lucrative scheme that requires massive documentation to prevent? And has anyone ever read to the end of the DHS document?
posted by lesbiassparrow at 3:53 AM on October 25, 2013 [2 favorites]

Maybe it is like the Jaffa Cake Tax thing?
posted by Just this guy, y'know at 3:57 AM on October 25, 2013

If love is a bolt from the blue
Then what is a bolt but a glorified screw
posted by Pyrogenesis at 4:09 AM on October 25, 2013 [3 favorites]

"international standards are not necessarily applicable" to the US — which is hilarious if you've ever sold anything into the US, aka "What are all these weird-ass numbering systems?"-land.
posted by scruss at 4:27 AM on October 25, 2013 [3 favorites]

As if it weren't confused enough already, now HSA is involved?

The die is cast.
posted by Kirth Gerson at 4:28 AM on October 25, 2013 [1 favorite]

I'm not even sure if there's a single-point here.
posted by Kirth Gerson at 4:29 AM on October 25, 2013

The terrorists have won.
posted by mr vino at 4:41 AM on October 25, 2013 [3 favorites]

Kenneth Gaburo's Maledetto (an experimental audio performance piece) has an extended discourse on the definition of a screw (starts around 2:40).
posted by idiopath at 4:45 AM on October 25, 2013

"conversely, to the screw, as a groovy shafted nail"
posted by idiopath at 4:46 AM on October 25, 2013

If it screws into pre-existing thread, I call it a bolt.

Exception: If it's small enough to be dropped and irretrievably lost somewhere within the overall apparatus, it's a screw. (see Machine Screw)

If it screws into non-threaded material, I call it a screw.

Exception: If a proper fastening requires a nut, but you're being lazy by just threading something into the material, it's a bolt. (See Lag Bolt)
posted by RonButNotStupid at 4:48 AM on October 25, 2013 [1 favorite]

There's totally guys whose job is to know weird shit in the tarriff schedules and help people (well, businesses) figure out what tariffs they'll be paying on their regularly-scheduled industrial imports. A friend of a friend has that job in Canada.

My understanding is that it's damn hard to simplify things, because a lot of the funny distinctions are due to harmonizing things with respect to a country's individual laws, laws of the point of origin, and various international treaties. But yeah, it might be worth the effort to try to develop a tarriff schedule that fits in a twenty page document...
posted by kaibutsu at 5:00 AM on October 25, 2013

I was all ready to agree with the "parallel sides, takes a nut" vs "tapered, no nut" distinctions, until the subject of machine screws came up. I commonly use little 8-32 or 1/4-20 jobbies with hex heads, and parallel sides, and I think it's about 50-50 whether I put a washer and a nut on 'em, or screw them into pre-existing threads on whatever it is I'm trying to assemble. (Optics research assemblies are a lot like tinker toys.)

But they're much too small and fiddly to dignify with the name of "bolt."

So I've decided that it's a screw unless it meets all three of these conditions 1) parallel sides 2) neither a slot nor a Phillips head (ie, there's no such thing a flat-head "bolt driver") 3) at least the size of my pinky.

I believe that should resolve the debate for everyone. I can circulate some plaster casts of my pinky if it would be helpful.
posted by OnceUponATime at 5:15 AM on October 25, 2013 [1 favorite]

Would you prefer a sex bolt or Chicago screw?
posted by delicious-luncheon at 5:22 AM on October 25, 2013

Is this a major issue?

The internet is an experiment to isolate the smallest possible piece of explanation. There is no sign of the possibly mythical "know-on" as of yet.
posted by GenjiandProust at 6:06 AM on October 25, 2013

Pinback: "First day of the semester, the shop teacher walks in and asks the class "does anyone know the difference between a nail, a screw, and a bolt?""

First day on the construction site, and the new Irish immigrant is getting some friendly hazing from the foreman.

"Say, man, what's the difference between a girder and a joist?"
"A..girder, an' a joist?"
"Yes, what's the difference."

The new guy ponders for a minute, muttering.
"A girder, and a joist. Hmm....I've got it!"
"Ok, what's the difference?"
"Girder wrote Faust, and Joist wrote Ulysses!"
posted by notsnot at 6:10 AM on October 25, 2013 [8 favorites]

10-32 for life, and also you are all wrong. All of you.
posted by aramaic at 6:27 AM on October 25, 2013

Aramaic, a buddy of mine posted on FB that he'd bought a whole bunch of 1/4-20 screws from some project, when the tapped holes were all, apparently 1/4-28.
I replied, "UNF! UNF!"

Uh, yeah, I'm just leaving.
posted by notsnot at 6:32 AM on October 25, 2013 [1 favorite]

Actually I heard the telephone sanitizers have a strong opinion on this vital issue.
posted by miyabo at 6:36 AM on October 25, 2013 [2 favorites]

I replied, "UNF! UNF!"

I presume that if he said he'd gone to the Harley-Davidson dealer and come back with 1/4-24 screws, you'd be all "potato! potato!"
posted by Kirth Gerson at 6:43 AM on October 25, 2013 [1 favorite]

It's really a mess. But in general in the erikoverse.

Bolts go into one threaded object. This may be through a hole with a nut on the other side, or into a tapped hole. Yes, this means many machine screws should be called machine bolts, but they aren't, because English.

Studs go into a hole and have another threaded object applied to them later (usually a nut.) Basically, they're bolts without heads. You screw or press the stud into an object, slide something over it, and bolt it down.

Screws are driving by rotation into material without tapping threads first. Self tapping screws are, therefore, screws.

Nails are driving directly into material.

Rivets are places into bored holes and peened over. Structural rivets, surprisingly enough, demand *very* close tolerances between the rivet and the hole, but very few people have worked with them, unless you've worked in the aerospace industry or motor racing (and it's dying out in the latter.) Pop rivets don't carry much load and tolerate much looser holes.

Pins are placed into bored holes, but are not fixed otherwise. They're used for alignment and to prevent shifting in the plane perpendicular to the pins. Like rivets, to work well, they need very close tolerances.
posted by eriko at 6:53 AM on October 25, 2013 [2 favorites]

The real question is: what's the difference between a self-sealing stem bolt and a reverse-ratcheting routing planer?
posted by kjh at 7:00 AM on October 25, 2013 [1 favorite]

If you thought that DHS document was spellbinding, it's just one of several dozen Informed Compliance Publications you can read today! Learn how to tell granite apart from syenite! Find out if there are special rules governing pencils with novelty toppers! The latter document points us to ruling NY D82043, which says, in part:

Item 16770 are Fright Writes Pencils. The samples you provided have an eraser in the shape of either a black cat, a witch or a pumpkin. The pencils are painted with the word "boo," and are either black with cartoon ghosts or purple with jack-o'-lanterns.

posted by theodolite at 7:02 AM on October 25, 2013 [2 favorites]

And the top is just the bottom after a vertical inversion!
posted by blue_beetle at 7:03 AM on October 25, 2013

I'm pretty much in the "screws are driven by their heads" and "bolts are secured by nuts", head, taper, size, notwithstanding. Both the third link (engineer explains) and pretty much the DHS agree that this is the essential difference. It's how it's used.

Usually, but not always, how it's going to be used determines its form. So something that is driven by its head and secures by pulling on what it's threaded into, is a screw. Similarly, something that isn't driven by its head, but rather secures by being pulled by a nut, is a bolt.

If you think about it, this is really the distinction that matters. It makes sense to define the terms this way, rather than by form, because the forms can vary in ways that make the different between screws and bolts ambiguous.

The archetypal screw is pretty much the wood screw — it's self-tapping, it's tapered, etc. And the archetypal bolt is pretty much the carriage bolt — it isn't self-tapping and it cannot even be driven by its head at all, it requires a nut to work, and it's not tapered.

But there are circumstances where you need a bolt to be a little like a screw. The biggest reason is that you can have a bolt where tightening the nut will cause the bolt to turn, so you need some way to hold the head stationary, so you make the head with a screwdriver fitting or a wrench fitting. But that makes it possible to be driven like a screw, where you're really tightening it by the head. That could well be a mistake, and it's not intended to be tightened that way! Similarly, maybe you want a bolt to basically be self-placing by way of being self-tapping and sort of self-threading ... not so much because it's going to secure via any threading in the material, just that it's convenient to get the bolt through the material that way, where you can then use a nut on the other side to secure.

So, again, if you insist on making the nomenclature distinctions on the basis of form, then you're going to have lots of ambiguity and people will disagree. But if you define it in terms of function, then it's straightforward. If it's driven by the head and that's how it's secured, then it's a screw. If it has a nut on the other side and the nut isn't permanently affixed, then it's a bolt and if you are using it as a screw, maybe you shouldn't be using it that way and it really is a bolt. If it's securing because the material is threaded and you necessarily are driving it by the head, then it's a screw. It may look like a "bolt" to you, but it's a screw.
posted by Ivan Fyodorovich at 7:04 AM on October 25, 2013

Yeah, lots of overthinking. A screw is turned to fasten. A bolt is held in place while a nut is turned to fasten. A woodscrew or self-tapping metal screw uses its own threading to cut the corresponding thread, a machine screw relies on a pre-threaded hole. If you don't know this, you will be a sad doobie hunting through the fastener drawers at the hardware store.

The confusion comes in for bolts that are used as machine screws, or machine screws that look like bolts - typically hex-headed thick-gauge fasteners. Also, the verbs introduce confusion - you can bolt a part on with a screw, and you can screw in a bolt. They're not really specific to the fastener they share a name with.
posted by Slap*Happy at 7:18 AM on October 25, 2013

I can't really understand how we can define a bolt as something tightened by a nut. Anyone who has worked on a car or any kind of modern machinery knows that there are many things that are turned by the head into tapped holes that are very clearly bolts. No one calls them screws.

If you took the same objects that are used in those tapped holes and used a nut on the other end, no one would hesitate to call them bolts. It makes little sense to have the exact same object change from screw to bolt just because you thread a nut on the end. No one believes their engine is held together with screws!

Sure, lag bolts are actually screws and machine screws are actually bolts, but that's just something we have to live with.
posted by ssg at 7:26 AM on October 25, 2013

No one believes their engine is held together with screws!

HURF DURF SCREWS ARE WEAK
posted by kiltedtaco at 7:28 AM on October 25, 2013 [1 favorite]

Anyone who has worked on a car or any kind of modern machinery knows that there are many things that are turned by the head into tapped holes that are very clearly bolts. No one calls them screws.

They're usually called fasteners in the shop manual. Because of the aforementioned confusion over hex-headed, thick-gauge screws, it feels more natural to call them bolts. They're still machine screws, or hex-head threaded fasteners if you want to come correct.
posted by Slap*Happy at 7:30 AM on October 25, 2013

(Now we will argue about hex-head machine screw vs. hex-socket machine screw vs. allen bolt.)
posted by Slap*Happy at 7:32 AM on October 25, 2013

I'm glad somebody is on the case.

Next, please write a detailed report on the differences between a bean and a nut, cuz so far the distinctions I have heard are bullshit.

Beans and nuts are the same thing damnit.
posted by Ad hominem at 7:41 AM on October 25, 2013

I do a great deal of automotive translation and British->American English-ification for a automotive company with multinational manufacturing facilities. Many source languages have NO DISTINCTION FOR BOLT OR SCREW--simply one all-purpose word for threaded fastener.

Now, these texts I work with are generally addressed to mechanics, body assemblers, and professional drivers who would not buy no sissy engineer's argument that head bolts are actually screws. I spend a lot of time looking at pictures trying to figure out what are bolts and what are screws. Sometimes I don't get any visual reference material, and then I just say screw it! It drives me nuts and really torques me off sometimes.
posted by drlith at 7:42 AM on October 25, 2013 [1 favorite]

We need a new word. A screw should always have a tapered cross section. It should never have a cylindrical body. A bolt should always have a nut. Well the words do exist, but they are hyphenated compounds: metal-screw, wood-screw. Unfortunately lazy language users started calling metal-screws bolts and hence we have this needless confusion.

(I am not a lexicographer.)
posted by bukvich at 7:44 AM on October 25, 2013

The technical engineering term for what everyone thinks of as a typical bolt is a hex head cap screw. You also have button head cap screws, socket head cap screws, etc. I think the distinction should be based on the verb bolt and screw. If it is possible to turn the fastener with the head then it is a screw (basically everything), if you have to use a nut (like a carriage bolt) then it is a bolt. This does present a problem with press in studs (technically self clinching studs) where they really should be bolts. The bolt definition should include that it is removable.

Thank you for reading this installment of mechanical engineering pedantry.
posted by TheJoven at 8:06 AM on October 25, 2013 [3 favorites]

I also ascribe to school of thought that if your into the machine screw nomenclature (#10-32, #4-40, etc) then it's a screw, otherwise it's a bolt(1/4-20, 1/2-13)
posted by TheJoven at 8:12 AM on October 25, 2013

NEXT UP FOR DHS AND METAFILTER:

When is an engine used to generate motive power a motor, and when is it an engine?
posted by ROU_Xenophobe at 8:21 AM on October 25, 2013

If it is possible to turn the fastener with the head then it is a screw

ASTM A325 and A490 would like to have a word with you.
posted by aramaic at 8:23 AM on October 25, 2013

A bolt is just a cylinder(oid) you shove into or through a hole. It may have threads for fastening and securing it in place, but the threads are secondary to its main job, which is to be a cylinder(oid) shoved into or through a hole. Think deadbolt, or bolt action rifle. The bolt is a cylinder(oid) shoved into or through a hole. That's all.

By contrast, a screw screws in. It may bite into material, or it may go into a threaded fitting, but it must be screwed in. The screwing is key to its whole function.
posted by Sys Rq at 8:57 AM on October 25, 2013 [4 favorites]

This is why i use pegs and nails; threaded fasteners are the devil's connectors.
posted by Mister_A at 9:04 AM on October 25, 2013

Scrolts.
posted by islander at 9:12 AM on October 25, 2013 [2 favorites]

"A bolt is an externally
threaded fastener designed for in
sertion through the holes
in assembled parts, and is normally in
tended to be tightened or released by
torquing a nut."

Can't you also torque the bolt to tighten or release the nut?
posted by ChuckRamone at 9:13 AM on October 25, 2013

Either way, I hope to never torque one or both of my "nuts" since they are not attached to a bolt and are not intended to be used like the nut and bolt and screw fasteners being discussed.
posted by ChuckRamone at 9:16 AM on October 25, 2013

If you use a wrench to fasten, it's a bolt. If you use a screwdriver, it's a screw.

Nuts to nuts.
posted by Pazzovizza at 9:24 AM on October 25, 2013

Just in time for the show.
posted by and for no one at 9:33 AM on October 25, 2013

islander solved it. This issue is resolved.
posted by chisel at 9:42 AM on October 25, 2013

I won't be satisfied until America is the only place in the world that gets struck by lightning screws.
posted by srboisvert at 11:48 AM on October 25, 2013

Scrolts.

No. Brews.
posted by Kirth Gerson at 12:14 PM on October 25, 2013

If it is a fastener that is a) threaded, and b) has a tapered end, it is a screw. Otherwise, it is a bolt. The shape of the head should be completely irrelevant.

Yes, this makes machine screws technically bolts, which makes sense to me since machine screws look a heck of a lot more like carriage bolts than they do any type of screw I've ever seen.
posted by zug at 12:42 PM on October 25, 2013

The difference is obvious. The threads on bolts are woven together.
posted by TedW at 12:48 PM on October 25, 2013

Scrolts.

I've heard you can get that cleared up with a medical cream now.
posted by FatherDagon at 3:27 PM on October 25, 2013

Any externally threaded cylindrical fastener is a machine screw. It can be referred to as a bolt if you use it in an application that mates it with a nut, but it can always be called a screw no matter how it is applied.
The "bolts" used to attach manifolds to engine blocks are misnamed, since they aren't mated with a corresponding nut..
posted by rocket88 at 3:30 PM on October 25, 2013

Here's a site that collected a whole bunch of other sources' definitions of bolt and screw, They start with dictionary definitions, then through Machinery's Handbook, the HSA paper, and the ANSI standard. Everybody except some of the dictionaries seems to go with the "bolts mate with nuts" distinction. They end with this:

Naturally there are exceptions to every rule as the Millwrights and Mechanics Guide 4th Edition observes: "These definitions obviously do not always apply, since bolts can be screwed into threaded holes and screws can be used with a nut." And several other examples come to mind: A toggle bolt consists of toggle wings, which is a special spring-loaded collapsible nut, and a screw--you always tighten the screw, but the assembly is called a toggle bolt. Stove bolts are essentially machine screws. Lag screws are sometimes called lag bolts. And fully-threaded cap screws are also called tap bolts.

posted by Kirth Gerson at 5:15 PM on October 25, 2013

"Scrolts.

I've heard you can get that cleared up with a medical cream now."

Thank goodness for the American DHS, providing mechanical treatment for every citizen in need.
posted by iamkimiam at 9:29 PM on October 25, 2013

Interestingly enough there are various ways of shaping the point of a screw.
posted by Confess, Fletch at 10:04 PM on October 25, 2013

Etymologically speaking, a bolt is a shaft or a rod. This is why we talk about a crossbow bolt, or the bolt of a door. A screw is a helical surface or volume, such as Archimedean screws or screws of paper.

So a bolt doesn't need to have a thread at all; and a screw doesn't need to be on the surface of a shaft. I suggest that all rod-like fasteners might be called bolts, but that the word "screw" is a useful term to distinguish bolts that exert pressure from either end (by the use of a nut or swaging or whatever) from bolts where the pressure is exerted by the thread on the bolt's surface, which runs within the materials to be joined. In those cases the mechanical force is internal; it really is being applied by a screw; and you couldn't achieve the same result with a smooth bolt capped at each end.
posted by Joe in Australia at 6:11 AM on October 26, 2013 [1 favorite]