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BSF and Whitworth "Across the flats " sizes.

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Andrew Tinsley07/10/2017 11:35:47
1181 forum posts

Am I correct in assuming that the spanner sizes for Whit and BSF are the same.? I realise bolt O/D will differ!)

I have plenty of AF and also Whit spanners. I must have used these on BSF size bolts without realising. A quick check on a couple of BSF bolts seems to indicate that the head sizes are the same as Whit, is this generally true, or have I just been lucky?

Andrew.

Hillclimber07/10/2017 11:45:26
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Yes.

Of course, you get this funny thing in which pre and post war 'declared' sizes are different because they reduced the size of the whitworth head to one size smaller to save steel. But the range of across-flats dimensions are the same, you only need one set of spanners for bsw/bsf....

Cheers, Colin

Edited By Hillclimber on 07/10/2017 11:46:10

Mike Poole07/10/2017 11:46:46
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2748 forum posts
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Whitworth and BSF use the same sizes but just forget the markings on the spanner, also if you come across very old Whitworth stuff it will have a larger head as they downsized the heads. This is why the marked sizes on spanners don't make sense.

Mike

Carl Wilson 407/10/2017 12:56:09
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I used to work on a type of aircraft that was all BSF and BA.

On detachments the toolkit carried was quite rudimentary due to space restrictions. Hence it would sometimes be necessary for me to be able to use a locally borrowed standard imperial or metric AF spanner/socket on a BSF bolt. Not ideal, but sometimes you have to urinate with the penis at your disposal.

So I needed to know what the AF sizes of BSF bolts were in order to find the closest metric or imperial size. Fortunately there is a formula and this is it:-

AF = Whit.dia x 1.5 + 0.125.

For a BSF bolt the Whitworth diameter is the thread core diameter and the BSF diameter is the thread crest, eg 1/4 BSF = 3/16 Whit.

This info is all based on BS47 (1948).
Andrew Tinsley07/10/2017 13:00:03
1181 forum posts

Thanks gentlemen!

That will save me buying BSF spanners. Probably not as easy to do these days!

Andrew.

Carl Wilson 407/10/2017 13:08:45
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Hey Andrew,

The caveat to the info I just gave you - which is sacred aircraft technician arcane knowledge to be guarded with your life - is take care. Some of the AF standard sizes are 1/32 larger than the BSF across flats dimension.

So for some of the sizes very infrequent use is recommended, ie to get you out of trouble. Even then use bi hexagonal socket or ring spanner with care.
Andrew Tinsley07/10/2017 16:17:47
1181 forum posts

Hello Carl,

Thanks for the caveat. I have some BSF spanners on order!

Regards,

Andrew.

Carl Wilson 407/10/2017 16:28:54
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671 forum posts
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No problem.
Anthony Knights08/10/2017 10:35:05
427 forum posts
185 photos

All my Whit/BSF spanners (which date from my motorcycle days in the early 60's) are dual marked with the whitworth size being one smaller than the BSF. eg 5/16 whit also fits 3/8 BSF.

Carl Wilson 408/10/2017 13:09:19
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671 forum posts
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Same on aircraft. Mostly Britool stuff, as bought by the MOD.
ega08/10/2017 22:13:56
1812 forum posts
153 photos

It seems that metric AF sizes also vary eg an M12 nut encountered today which takes an 18 mm spanner rather than the normal 19.

Carl Wilson 408/10/2017 22:16:45
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671 forum posts
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Was it Japanese? That is common on Japanese motorbikes apparently. Also I once worked on a very large winch that was built in Romania, the fasteners were like that too.
Mike Poole08/10/2017 22:56:48
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2748 forum posts
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Volkswagen also use odd sizes, you will find a use for that 16mm as well as 18mm, what you thought was torx may also be spline drive, yet another flavour of multipoint drive. You will never have enough tools as long as they keep inventing new drive standards. Once we just had flat blade screwdrivers then Philips came along then Pozidrive then supadrive then square,

Mike

ega08/10/2017 23:11:09
1812 forum posts
153 photos
Posted by Carl Wilson 4 on 08/10/2017 22:16:45:
Was it Japanese?...

That M12 nut retains the wheel on a Creusen bench grinder.

Mike Poole:

Some time ago I came across an "Allen" screw with a five-sided recess. This design may be intended to be tamper-proof.

Mike Poole08/10/2017 23:14:28
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2748 forum posts
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iPhone use 5 sided Allen type fasteners, it seems a pointless exercise as almost within hours the tool to fit will become available.

Mike

David Standing 108/10/2017 23:41:38
1289 forum posts
48 photos
Posted by Mike Poole on 08/10/2017 23:14:28:

iPhone use 5 sided Allen type fasteners, it seems a pointless exercise as almost within hours the tool to fit will become available.

Mike

They are more like a five sided Torx fitting than an Allen fastening.

Mike Poole08/10/2017 23:47:01
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2748 forum posts
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Have to get my magnifying glass out and have close look.

Mike

Hopper09/10/2017 01:07:29
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4817 forum posts
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Just as an aside, I read somewhere that Mr Whitworth made his original across-the-flat dimensions so oddball because they were designed so the hexagons could be machined out of the limited available range of round bar stock in those days. So the hex dimensions are really based more on the across-the-points measurement and the spanners were made to fit the oddball flat dimensions that resulted.

What would we expect from the guy who designed and built muskets for the American Civil war that had a hexagonal shaped bore down the barrel and would fire only the supplied special hexagonal Whitworth bullets? You think it's a headache having a Whitworth bolt and nut and only AF spanners!

Marcus Bowman09/10/2017 07:47:32
166 forum posts

Once upon a time, MEW or ME published a table showing equivalences across W, BSF and Metric bolt heads, Sadly, I can't recall the issue.

Marcus

Brian G09/10/2017 08:12:03
716 forum posts
29 photos

This table is very useful at a pinch, giving BA, metric, AF, BS and WW sizes from 10BA up to 60mm **LINK**

I think the worst thread size in this respect is probably M12. Depending on standard, it requires 17, 18, 19, 21 and 22 mm spanners. I am curious about one thing though, why does my Britool set have 9 and 15 mm spanners when, as far as I can find, they shouldn't fit anything?

Brian

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