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MG Midget and Sprite Technical - Screw it or bolt it? Water pump.

This isn't really a water pump question, but having spotted a slight weep from the spindle, I had to pull the w/pump on the midget today.

Normally it's held on by 2 long and 2 shorts bolts, the long ones being BH605151. However, whilst the short ones were indeed bolts, the long ones were screws.

If yesterday I hadn't completed my order with Sussex, I'd have added two of the correct bolts to the order, and used them. But since I don't fancy paying more in postage for the bolts than the cost of them on a small order, don't have any spare bolts that length either, and nobody around here stocks imperial anymore, I'm going to shove the screws back in.

My reasoning is that whoever used screws instead of bolts, didn't cause a problem in this application.

So not being a certified engineer myself, who can tell me when it should be a bolt, and when it should be/could be a screw?

In this application, which is dowled, where either bolt or screw, it's being tightened into a blind thread, does it matter, and why?
Lawrence Slater

I'm guessing stretch might be a factor on a screw, when using higher torques.
Lawrence Slater

Lawrence, . So what are calling a screw? as according to that explanation you're unlikely to have bolts in there due to the difficulty/impossibility to get at any nuts on the bolts.
David Billington

I thought that a screw was threaded over its full length, whereas a bolt has an unthreaded shank?
Dave O'Neill2

Shouldn't believe everything you read, just because someone has written it down! That link is rubbish, and clearly wrong.

i.e. : to quote "screws that are unsuitable to use for bolts are unsuitable because they cannot be used with a nut."

A machine screw may be designed to screw into a component, but that does not prevent it being used with a nut.

and: "Nearly any bolt with a common head can be used as a screw by tightening it by the head into a tapped hole."

except of course, being a bolt it will have a non-threaded shank which will prevent it being tightened fully into a tapped hole.

The design and uses for a bolt and a machine screw may be correct, but his explanation of the distinction is just rubbish!
Guy W

I didn't read it, as the link wouldn't work for me.
Dave O'Neill2


You either didn't read the whole thing or missed the
"Screws that are unsuitable to use as bolts are wood screws, lag screws, sheet metal screws and all of the screws that fall into the category of self tapping screws." and the bit that you quoted was in full "The afore-mentioned screws that are unsuitable to use for bolts are unsuitable because they cannot be used with a nut" which refers to the first quote. I expect spire nuts blur the definition I linked to with self tappers.


We may also be getting into regional differences in usage here such as UK versus US. My local engineering supplier refers to fully threaded bolts or screws as set screws or bolts, whereas ones with a plain section are just screws or bolts. If I really want to make sure I get what I want you then have to specify whether socket cap, countersunk socket, pan head pozi, socket button head, cheese head (always slotted I think), etc etc.
David Billington

Dave, I did read it all, but the sequencing of the paragraphs makes it unclear what is being referred to when he says "the aforementioned screws" as he inserts a paragraph that describes (machine/ set)screws after the one referring to wood screws etc. At the very least, its poorly written and the juxtaposition of the text saying screws cannot be used with a nut is misleading.

The other reference saying bolts can be used as a (machine/set)screw would depend on the application and is frequently not true.
Guy W

Just coatem with antisieze. If you think they are usable. Cant you get those at a hardware store if you want bolts? Just asking. :-)
Steven Devine


Yes maybe not the best written article but it basically said what I experience as usage in my area. I frequently buy "BZP HT sets" (Bright Zinc Plated High Tensile (8.8) sets (fully threaded)) from my local supplier and these are called screws as they're not supplied with nuts but you can buy nuts for them, they also sell XOX bolts, can't remember what the XOX is as I rarely buy them. The XOX bolts come without nuts in the bin but for each one you buy they provide a nut included in the price. It doesn't help that some types of fastener don't have standardised dimensions as I've found with some type where the plain unthreaded section of a bolt or screw is left to the maker.
David Billington

England is different to US - we used to have local stores but now they've all been lost to US globals selling things like packets of only the most popular sizes of bendy/snappy screws

I think the invasion is to take over our water and ship it over to the Hoover so you can have grass lawns in the desert
Nigel Atkins

Yes Ive noticed all the big box store owners seemed to have acquired your merchants shops. Thats not a good sign you know! :-(
Steven Devine

Like steven,

I cant relate to not being able to go to a hardware or home improvment store and just buying replacements....granted I always have to 3-4 stores to find what im looking for

As to reusing the old.hardware.... I dont see why not, as long as the threads are okay...its not like its holding specialized torque like a cylinder head stud, I dont even use a torque wrench... just snug it up

If was to worry about anything it would be the gadket material and the type of glue used to seal the pump ont the block

But I always use the old bolts/screws... unless there damaged in someway....aka rust

Considering this is water.and.coolant... maybe a small smear of anti seizure compound to keep the bolts from rusting in the block....I do this with my thermostate housing hardware and it makes changing stats a snap and the hardware completely reuseable

Prop and the Blackhole Midget

Amazing thread (pun intended) drift. lol.

I've always understood it to be as O'Neill2 describes it. A screw is threaded full length, and a bolt has an unthreaded shank.

The orignal specification is for "BOLTS" I gave the part number of the bolts in question. BH605151. BOLT, HEX HEAD, UNF, 5/16" X 1 7/8".

Back to the original question. :).

--- "So not being a certified engineer myself, who can tell me when it should be a bolt, and when it should be/could be a screw?

In this application, which is dowled, where either bolt or screw, it's being tightened into a blind thread, does it matter, and why?" --- .

I also added that I think in higher torque applications, a bolt(unthreaded shank) would be more suitable, since presumably fully threading the whole length, is likely to cause more stretch/potential to snap.

I also wonder if it's engineering practice to use a bolt, where, as in the pic below, the item being clamped is much thicker than a washer. Is there a thickness of clamped item/thread ratio or something, that determines when you should use a bolt?

PS, I've already seen that link, and think it good but a little contradictory. Also, it didn't specifically answer my question. But I'll gladly nick a pic from it. :).

Lawrence Slater

Whoops. That should have read,

"I've always understood it to be as 'DAVE' O'Neill2 ---- "

Just using O'Neill2, looks rather unfriendly.
Lawrence Slater

Size for size, bolts always look sort of stronger - as if for heavier duty applications. Or such that the unthreaded shank bit is designed for lateral loadings and more accurate alignment of whatever is being bolted up.

But maybe its just for cheapness with less thread cutting to do?
Guy W

The usual definition of a "screw" is that it is fully threaded whereas a bolt is only threaded for part of its length. The terms "setbolt" and "bolt" are often used in a similar manner. Unless there are issues of the unthreaded portion bottoming out it shouldn't matter which you use. I don't think the head types alter the definition although I would regard a slotted head fastener as a screw. I just think it's a case of casual use of the terminologies.

In engineering terms a screw is able to take a nut, so that eliminates self-tappers, wood screws and so on.

GraemeW (Kent!)

Well I'm truly surprised.

Plenty of discussion about the difference between a screw and bolt, -- which wasn't the question --, but apart from myself and Guy, nobody else seems to have any kind of answer to the question I actually asked.

I agree about lateral loading and accurate location with bolts Guy, that's why I mentioned the dowls in my first post. -- Hinting that without the dowels, I'd have thought bolts were more neccessary.

Come on you engineers. Surely one of you must be able to point me to the particular purpose for a bolt, as opposed to a screw?
Lawrence Slater

Bolts are generally used when they also serve to locate the items being joined - e.g. bellhousing to backplate - set screws are generally used where the items being joined is relatively thin - e.g. engine mounting rubbers.

In either case the bolt or set screw may go into a blind threaded hole - e.g. the block, an open threaded hole - e.g. starter motor or pass right though drilled holes - e.g. bellhousing.

It is unusual to find set screws used other than in the shorter sizes for the simple reason that it is cheaper to thread only the required length rather than the full length - since the fastening is as strong as its weakest point, strength isn't the issue.

However, in the aftermarket, set screws are common in longer lengths since it enables them to be used universally.

Specifically - the Midget water pump can use either - it is located by dowel - the later MGB pump is not dowelled so needs bolts.
Chris at Octarine Services

You will also find that bolts give a better fit and allow less lateral movement to the part being fitted than set screws.

And over, a "set screw" equates to your "grub screw". The plot thickens.

Gryf Ketcherside

Thank you Chris and John for those additional answers. It would appear from the answers then, that not many people have given this much thought. -- Or it could be, that those are the only people interested in answering ;).

A little more research, has confrimed my thinking that you would use less torque on a machine screw, than on a bolt of the same thread diameter.


It says,
"The means of calculating the suggested tightening torque is the same for machine screws as it is for bolts. The values are just smaller."

So you wouldn't swap main cap bolts -- 7/16" x 2.3/4", for the same size machine screws, even though the cap/block is doweld.

Lawrence Slater


I think he is referring to physical size of screws being typically smaller than bolts and not that screws can take less torque than bolts. Another of his articles on bolts gives the same formula and no reference whether a plain section or fully threaded fastener.

Also he states "It takes more force to stretch an SAE grade 8 bolt than it does to stretch an SAE grade 5 bolt because of the greater material strength". I think that is misleading as fasteners are not normally stretched to yield and so with the same conditions the same torque will stretch both equally due to the modulus of elasticity being effectively the same for all normal steels. While the grade 8 is stronger it will take more force to stretch it to the specified 75% of yield strength.
David Billington

Gryf - no the plot doesn't thicken, just another example of the US calling things by the wrong name! 8-)

I suppose to be totally unambiguous worldwide I should call them hex headed set screws ( or cap headed or whatever).

Lawrence - the issue is one of strength and clamping force, the set screw would stretch more than the bolt to achieve the same clamping force so technically it may well be possible to fit a stronger set screw of the same dimensions and achieve the same clamping force as for a bolt. However the cost would probably be higher.

In many cases where both ends of a fixing can be accessed - e.g. conrod bolts, bolt stretch is used rather than torque to measure the clamping force.
Chris at Octarine Services

Just posted an answer to David, but deleted as Chris answered again.

Lawrence Slater

Hi David. He does differentiate when talking about screws though, so isn't he implying what Chris has just said.

Chris, so my guess (2nd post) about stretch is relevant, and that pretty much answers all my questions. Ta very much.

Doesn't seem to have interested many here, but interested me. :).

Now about the difference between a bolt and a screw.

Shovel or a spade? I call a spade a spade, what do you call it? ;).

And since this involved water pumps, who supplies the best for the A-series engine, and how much?
Lawrence Slater

Lawrence, I have a new one that I don't need.
Dave O'Neill2


In what way does he differentiate between the two as in the 2 articles he wrote, one for screws and one for bolts, he uses the same formula and factor for the same conditions.

I'm currently thinking that the clamping load won't change whether you use a set screw or a bolt if you do both up to the same torque, the only thing that will be different is that the set screw will stretch more and therefore rotate through a greater angle before you achieve the desired torque, that's assuming the bolt plain shank is nominally of the thread OD.

David Billington

Stretch must be proportional to length and diameter. Since the bolt has a part of its length at the major diameter, that proportion will stretch less than the equivalent portion of a screw which has a minor diameter over its full length. Not that the degree of stretch is relevant in Lawrence' water pump fitting question.
Guy W

David. Because in the piece about screws he says, -- "The means of calculating the suggested tightening torque is the same for machine screws as it is for bolts. The values are just smaller."

Saying that the values are smaller for screws, is to say that there is a difference between bolts and screws, is the way I read it.

I agree Guy. The water pump isn't tightened enough to even begin to stretch the bolts/screws I wouldn't think. It's in higher torque situations I was thinking of, such as main caps etc.
Lawrence Slater


Technically any load on the bolt or screw will stretch it, no matter how small, so one needs to differentiate between permanent and elastic deformation. Any item stretched within its elastic limit will return to its original dimension when the load is removed, if stretched beyond the elastic limit it will become permanently deformed, permanently stretched, but it will still exhibit elastic behaviour and return to a shorter length when the load is removed albeit longer than it started out at as a result of plastic deformation.
David Billington

Yup I see that David.

Dave, sorry I missed your post. I've already bought a pump. But ta anyway.
Lawrence Slater

This thread was discussed between 15/01/2014 and 17/01/2014

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