55 degrees or 60 degrees

Question that suddenly intrigued me whilst sipping a glass of red over dinner this evening.

I wonder why Joseph Whitworth settled on 55 degrees and not 60 as was taken up later on by many other sections and standards.

Come to that how did 47.5 degrees become the standard for BA threads?

Didn't these people talk to each other?

Comments should be interesting!

Len. P.

Len,

When Joe was busy sketching his ideal thread we were not talking to the French, we couldn't understand the Swiss and the Americas were still overrun by buffalo so he had it all to himself for years. Then along comes communication and globalisation and we find all these variants.

The problem with threaded parts is that they last too long so we can never get to a point where a particular thread can be cast into the abyss. Somebody always comes along and wants one more nut or screw to finish his refurbishing job!

Hence today we have 47 thousand different threads scattered round the world and none that can be forgotten and only a very few that can even start to interchange with each other.

It nevertheless keeps many thousands employed

Good eh.

Bob D

I find it amusing that pipe fittings here in France are specified by the (approximate) diameter of the thread, e.g., 12mm, 20mm, 26mm. But they are really BSP threads with a 55 deg. angle. A case of the British beating the French but the French not admitting it

I once owned a vintage Morris car. The engine used French Automobile thread (metric) but nuts and bolt heads were to Whitworth sizes!

Russell.

I have the Morris, a Minor Series MM and for years I thought a lot of the (engine) threads in particular were Whitworth - but found out a few years ago they are Metric! shock horror!

Why not use standard bolts (metric or whitworth) rather than go to the expense of making 'specials' !

Alan

Alan,

The old Moggies sure had a different thread but my father in law, who spent a lifetime in the motor trade always told me that Morris, and for that matter Wolseley, nuts and bolts were 1tpi out to British standards in order to boost parts sales. If you stripped a thread or lost a nut it was off to the Morris distibutor for spares sold at a premium. Gold plated Daimlers that Lord Nuffield ran around in didn't come cheap.

Mind you, Old Cyril always referred to M6 as 1/4" Metric and M8 as 5/16" Metric. He never really accepted the need for another thread.

Bob D

I almost said, it's down to the bastardisation of threads that makes the money!! Where there is need, there is money!!! However, I don't know why the 55 and 60 degree thread angles are why they are? I am sure there may be a mathematical answer somewhere? I am also sure that if you have a design that is wanted and needed(required) by lots, then make a thread to suit!!! You will always make money....

cynical I know, but.......?

Hi Alan and Robert (especially)

Have a look at the document with this url: - www.mgcars.org.uk/imgytr/pdf/enginehist.pdf - it expalins the mystery of the Moggie threads and bolt heads - round about page 6.

Keith

When you see the thread forms and understand the maths, the 55 degree form is actually superior when you take into account shear strength etc etc. It's metric that took the slightly inferior route in the name of simplicity. The B.A thread form only has a 47 1/2 degree angle because of the very generous rounding at the crest and root and is based a metric thread in that 0 B.A. has the same pitch and diameter as a M6 metric coarse but the thread form is different. I'd recommend the Workshop Series Book by Tubal Caine on Drills, Taps and Dies where it's all explained.

When you see the thread forms and understand the maths, the 55 degree form is actually superior when you take into account shear strength etc etc

I don't know much about these things but I do know that Whitworth was a bit of a nutter for mechanical perfection

He manufactured a rifle for the British army with the "perfect bore twist" for the flight of the bullet which the Confederates used to great effect

The British army of course purchased rifles from a cheaper manufacturer

http://en.wikipedia.org/wiki/Whitworth_rifle

Whitworth worked for Henry Maudslay in his early careeer and learned much from the great man, including the use of fluted taps to cut standardised (but not universal) threads. He also learned the merits of interchangeability from the work of Maudslay and Brunel on the Portsmouth blockmaking machinery.

Chris Trice is correct when he says that the 55º gives a stronger thread and at the time cast iron was very widely used as quality steel was expensive (thread standard established 1841 only just after the success of the Bessemer converter). As we know CI is weak in tension so threads had to be as strong as possible, hence 55º was established as the best compromise over many years of experiment. This prevented shearing (stripping) of the internal CI threads. The Whitworth standards effectively became the British de facto standard when it was adopted for use on the railway system here. Metric threads were standardised much later.

In the USA, Sellers developed the thread standards which were eventually to become the Unified System. He used the Whitworth system as a basis but adopted the 60º angle. Various metric systems were established at that time but were unified at the 1894 Internation Congress in Zurich (not France) using the American 60º standard. Metric standards were finally established at the ISO conference in 1947.

Sorry for the long post and even that is an oversimplification, there is much more to it than that, but it gives an outline as to why we are where we are now.

Best regards

Terry

Edited By Terryd on 20/04/2012 06:25:50

One thread that I found while looking for something else, conduit thread, these are 80* so that they could have a reasonable pitch, but as they are on thin tube, the thread needs to be shallow. Ian S C

I always thought that when Mr. Whitworth decided to standardise threads he started by measuring all the threads he could get hold of, and then "averaging them", and so arriving at 55 deg. and the pitches. Maybe just a story tho'.

No. It goes back to the early days of the Bullnose Morris. Morris made the early engines for these cars under licence from the French Hodgekis (sp. ?) company and they had French Automobile threads throughout. The chassis used the standard practise at the time of using Whitworth threads so, as garages at the time would only have W.W. spanners they decided to use these metric threads but with W.W. head sizes.

The practise continued for some time.

Russell.

I understand that many modern cars use whitworth threads for the cylinder head bolts as they are less likely to strip in an alloy block.

Joey

I've still got some bits left over from my M1000 days, a spanner for head bolts 19/32" AF from memory. Was thinking yesterday how much I would like to be able to buy one now, for the same sort of price as they were then.Most cars etc. in those days had a right mix of threads and spanner sizes, check Land Rovers.

Keith,

Thanks for that link on the MGs. Perhaps Cyril was closer than I thought when he called them 1/4" Metric!

The bit about the origins of SU carbs was worth finding as well.

Gordon,

I can't agree completely about the assorted tools on old uns.

I was raised on Fords and although I don't remember the exact detail I believe Ford only supplied 2 spanners, a plug spanner and a wheel brace on the end of the starting handle in the tool kit but claimed that you could do a full engine strip without any further tools.

I'm sure somebody will be forthcoming with the detail

Bob D

Edited By Robert Dodds on 20/04/2012 17:57:44

Bob,

I don't know about the 60s, but my Cortina 'Mk V' certainly put some extreme demands on the toolkit. I remember changing something (it may have been a waterpump) and it had three different sizes of fixings for three bolts. If I recall; some parts were held on by a mix of cap screws and hex fixings.

My more modern Vectra is almost all selected from a sensible range of metric sizes - almost all you need is a standard set of hex keys, sockets, and a very few torx sockets, most of which go unused.

Neil

Keith - very interesting link about MG and Morris!

I remember years ago, on the Morris, I removed a wheel and found a very stretched stud, the thread angle on this must have been about 25 deg in one part (although it hadn't broke!) I saved it for years as I couldn't believe it had stretched that far!

It is now lost, but you boys would have loved to see a photo of it!

Alan

Neil,

The era I was thinking about when limited tools were needed was spanning the 30's to 50's when the model Y and 7Y with 933cc and 1072cc side valve engines were the popular car of the day. After that it started to go down hill and I had abandoned Ford after the Mk 3 Cortina in favour of Rootes Group models which offered much more comfortable leather for your money than Ford did.

Bob D

De Havilland I seem to remember used metric threads on their engines, but in my collection of old books, I have "Care and Maintainance of the 120H.P. D.H. Gipsy III Areo Engine", they get round the problem, the thread type is not mentioned, and spanners are refered to by a part number, the length of the spanner giving the required torque for that particular fitting.When refitting connecting rods to crankshaft, the big end nuts should be tightened upas tight as possible, using spanner Part No. T.1900-1. Under no circumstances shoulany other type of spannerbe used as this tool has been specially designed to avoid any unnecessary stress being applied to the big end bolts.Nuts should not be slacked back to get split pin in. Should slot in nut go beyond the hole, the nut should be faced off slightly to bring the next slot in line.

It also says Fuel: Good grade automobile fuel. We have here got back to that for some aircraft types in NZ. Ian S C