Name that screw

The convention is that a bolt is not wholly threaded along it's length. If it is, them it would normally be called a machine screw.

It's not normally considered 'coincidental'?

I have always believed that bolts were designed to assist in location, apart from being cheaper to make than machine screws.

Unfortunately the wikipedia entry for Machine Screws is ASME / US based.

Fulton Mackay?

Never worked on a car engine? You'll find plenty bolts on that - only part threaded - that go into a casting. So no nut. The non threaded part is larger than the thread so helps to locate what it's fixing more accurately. The reason - apart from strength - that a bolt is used.

Pan head is perhaps the most common none CSK type. I'd guess the OP is looking for a larger diameter head than that. The sort used for fixing sheet materials of various types.

It is purely "incidental". The different threading on nuts and bolts is a direct result of the different functions they were designed to perform.

Nobody decided to produce fasteners where the threading only went three quarters of the way along and then worked out what they could be used for - joining components through unthreaded holes which could then be tightened with a nut.

What they needed to produce was something which could join components through an unthreaded hole which could be tightened with a nut - which is what we call a bolt.

The fact that the threading on this fastner doesn't need to cover the entire length is purely incidental. The bolt could work equally well if it was threaded over its entire length,

What defines nuts and bolts is the way they're used, not their appearance

My previous explanation was entirely OTTOHM. However logical sounding definitions. once encountered, seem to stick in the memory..

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

Machinery's Handbook

Eh ? Its precisely the opposite. Provided the nut and the bolt head are big enough there's no necessary connection whatsoever between the bolt diameter and the hole through which its put, Even more so, if oversize washers are also used at either end.

michael adams

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The fact that they may look like bolts is immaterial. They in fact function as screws

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

Machinery's Handbook

Calling a particular machined fastener a "bolt" doesn't in fact make it any stronger than an identical machined fastener that someone else, more correctly I would suggest, calls a "screw".

michael adams

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I wouldn't call two entirely different functions 'coincidental'.

I don't get your point. Bolts are generally cheaper to make and many bolts I have come across are intended to provide an element of location. A good example is attaching two castings together. There need be no nit where one is threaded and the other plain and a close fit to the bolt.

Which I have already stated is USA centric.

Ah, I can see the problem.

I can assure you a bolt, with a non-threaded portion can be used for location, whereas the indeterminate true diameter of a thread is going to be a sloppy fit.

I can assure you a bolt can be used in castings and made captive without a nut. As Dave has said it seems you haven't worked much on cars.

There is usually a very strong correlation between the hole size and bolt/screw. Anything else would be considered a bodge.

I was in steelwork engineering for most of my working life, so what do I know.. i didn't say anything about the thread.

It's very simple really and there's nothing USA centric about it.

A bolt is something you push through a hole with a thread on the end. That's it. It doesn't move after that. You shove a nut on the other end and you turn the nut to tighten the bolt.

A screw is something you er screw into a threaded hole until you can't screw it in any further.

Spot the difference ?

They're screws not bolts.

Like Dave and all the other "experts" you're understandably confused because they do admittedly look like bolts. However that's only going by appearences

However like Dave, when you were screwing these "bolts" into these cylinder heads did you never stop and think "hold on, if I'm screwing these bolts into this cylinder head rather than just pushing them in and tightening them on the other side with a nut, like I do with all other bolts, why are they called "bolts" and not "screws" ?

But at least it *is* possible bodge with genuine bolts whereas it clearly isn't with the kind of nuts which you insist on calling bolts based solely on their appearance.

michael adams

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Hmm. So why do we have nuts and bolts if a bolt describes the whole thing?

Eh ?

Where does "a bolt" describe the whole thing.

You push the bolt through the hole and then place a nut on the end, and then tighten the nut.

If you don't screw a nut on the end and tighten it, then the bolt may possibly work loose and eventually fall out.

Admittedly what you call cylinder head "bolts" don't have nuts. But that's only because they're screws and and not bolts.

Whereas cylinder head "studs", which are first screwed into the head, then do then function as "bolts"; as they don't move any further, and are secured by cylinder head nuts.

michael adams

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Only in the uninformed world.

A bolt doesn't have a nut. It can quite easily be screwed into a casting or similar.

Quite, like a machine screw.

The purpose of a bolt or machine screw is for retention. I put a bolt into a threaded hold and can screw until I can't screw it in any further too. That would be the point of a bolt too.

Yes, a bolt has a plain shank, sometimes used for location and a machine screw is fully threaded, aprt from say the top 1mm or so.

They would be called bolts in the UK and I would purchase bolts; where a portion of the bolt is plain and not threaded.

I'm not an expert any more than you. You go to a supplier of fasteners and ask for machine screws or set screws and you'll get a fully threaded item. Ask for bolts and there will be a plain part. If critical it would be prudent to find out how long the plan part is. I believe there is an ISO standard.

You are now saying a cylinder head bolt is a screw because retention is not by a nut. This is in the face of centuries of UK tradition.

I think you're getting confused with nuts and bolts. I have never called a nut a bolt or visa-versa.

This might assist:

YMMV

I have plenty of hexagon-headed bolts where the thread goes right uup to the head though.

Is a cylinder head bolt, a bolt or a machine screw ?.

Possibly this may help clarify things.

Functionally its a screw, as its screwed into a thread; rather than being secured with a nut as is a bolt.

The fact that people mistakenly refer to them as "bolts" based solely on their appearance rather than on their function is unfortunate.

Whereas cylinder head "studs", which are first screwed into the head, then do then function as "bolts"; as they don't move any further, and are secured by cylinder head nuts.

michael adams

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Oops ! Make that screwed into the block. Doubtless other posters will be along in hours to come, to point out this error.

But you've said a bolt is only a bolt if it has a nut. No nut, it's not a bolt but a machine screw.

What *is* the correct distinction between a bolt (which is intended to be screwed into a nut, sandwiching something between bolt head and nut)) and a machine screw which is designed to be screwed into a larger frame? I presume both a bolt and a machine screw of a given diameter and thread pitch can be used interchangeably in either role.

Indeed. So if cylinder head bolts don't require nuts then they're not bolts but screws; as I said .

As to the precise definition of machine screws as against any other type of screw, say set screws, I have no strong opinions either way. The generic term "screw" will suffice.

BTW you seemed to have overlooked my rather unfortunate error in saying that studs are threaded into cylinder heads; rather than what I should have said, threaded into cylinder blocks.

michael adams

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