Following on from my last query re speedos and drives, what is the threadform of the front wheel speedo drive used where it screws into the hub? Most likely some obscure thread, but the nearest tap available now (as far as I can tell) seems to be M18 X 1, ie a Metric Fine thread. Anyone confirm or better that?
Thanks.

NB Ron Pier, seems my speedo drive is the correct rotation after all, just a bit tight against the brake arm boss..

email (option): kevin.sandford@ntlworld.com

As a tip for anyone looking for M20 speedo drives at swap meets etc. If you turn the drive dogs in a clockwise direction...The output drive for the cable should also be turning clockwise.
Kev I take it you have moved away from M20's here and you are talking about the thread on the angle speedo drive, as fitted to most other girder bikes? Ron

email (option): ronpier@talk21.com

>>>>take it you have moved away from M20's here and you are talking about the thread on the angle speedo drive, as fitted to most other girder bikes?<<<<

Yes, the older type which screws into the brake plate rather than the later more common type of Smiths speedo drive.
Any idea on the thread, Ron?

email (option): kevin.sandford@ntlworld.com

OK Kev. I just wanted to clear that up, in case anyone has one to measure.
I haven't got any thread gauges, but I'll see my engineer mate next Tuesday and give you an answer if no one beats me to it. Ron

email (option): ronpier@talk21.com

Kev. I just measured an old speedo drive I have here. I make it 18mm x 26tpi. Strange how these metric measurements crop up now and then. Ron

email (option): ronpier@talk21.com

I think that we've discussed before whether the mounting studs on 'Chronometric' speedo heads were BA or metric.

The Smiths instruments made under Jaeger patents were designed by Jaeger in France and originally produced in the UK by Jaeger due to British import tariffs on goods made in France.

It would seem logical that the Jaeger company would have used the same thread forms as in their other production facilities and that Smiths would have taken over this tooling and retained interchangeability.

I knew the attachment studs on the speedo heads are not 1/4", but I always thought they are 0BA. What is the metric equivalent? M6?

Ron

email (option): ronpier@talk21.com

Yes Ron. 'Radco' states: "No.'0'BA is, for practical purposes, interchangeable with 6mm, although thread angles differ"

I've never been able to tell the difference.

The speedo drive might not be 26 TPI but a 1mm pitch..that equates to 25.4 TPI. It seems more likely with an 18mm O.D. Regarding 0BA/6mm x 1 interchangeability, although an Imperial Thread BA was based on a metric thread form. BA thread angle is 47.5 degrees, 6mm is 60 degrees so a 6mm nut on a 0BA thread might be a bit slack, particularly if there is some wear on the BA thread....Ian

email (option): ian@wright52.plus.com

Hi All, don't exactly know what the tread on the speedo head studs are, but normally a M6 nut has a 10mm hex, but the original nuts on my speedo's are slightly bigger, like 10.5mm is that a normal imperial size?? 1/4" for reference is 11.3mm.

Cheers,

Lex

email (option): welbike@welbike.net

email (option): wd.register@gmail.com

Thanks Jan! now I'll have to find my 0BA tap and die!!, know I have 1,2.3.4 and 5.

Does your Jeager speedo have a pointy matt chrome bezel??

Cheers,

Lex

email (option): welbike@welbike.net

email (option): wd.register@gmail.com

My dull chromed nuts measure up at 10.47mm AF but it's not unheard of to mix and match the across flats sizes. They wouldn't have expected the owner to have a metric spanner.

Later Norton Commando engines have a number of unified threads but they retained the Whitworth spanner sizes (presumably for access reasons).

I should stress that I have no axe to grind re. the sizes but bearing in mind the French ancestry of the Jaeger, metric wouldn't seem any more out of place than it does on the spark plug threads (18mm on a real motorcycle ! )

Thank you Ron, I also measured it at 18mm and 25/26 tpi making it M18 X 1.
I wanted confirmation as it seemed unlikely that BSA would have used a metric threadform, but there you go.
Regarding the perennial M6/0BA debate, some combinations of nut and stud are too slack to be of any use, and others need force to fit.Either way it's bad practice, and all the while BA items are available there's no need either.Sermon over..

email (option): kevin.sandford@ntlworld.com

Hi Kev..I agree with your final sentiments..'mixing' threads is not good practice and shouldn't be done..In the 'engineering game' you would be regarded as some kind of retard if you carried out such activities.. ...Ian

email (option): ian@wright52.plus.com

The Dutch Army 1966 Triumph 3TA had the following threads: WW, BSF, UNF, BA, CEI and metric...
Great bikes nonetheless, there are still a lot running around, see http://www.triumph3ta.nl/Engels/indexengels.html

email (option): viaconsu # planet dot nl

Ian Wright

Hi Kev..I agree with your final sentiments..'mixing' threads is not good practice and shouldn't be done..In the 'engineering game' you would be regarded as some kind of retard if you carried out such activities.. ...Ian

email (option): britool51@hotmail.com

I hear what you say Robb! But I still make this speedo drive 18mm. I will however give it to my engineer mate on Tuesday for his opinion.
From memory! There are also at least two nuts on an M20 that require a 20mm spanner! I think one of them is the nut that holds the valve lifter in the cylinder. Ron

email (option): ronpier@talk21.com

Hi Ron..Your right, though I think Robb is refering primarily to threads .The other is the timing side mainshaft nut. Also not forgetting the 14mm thread for the spark plug and the front brake adjuster lug on the forks, the Metric drive side main bearings, main gearbox bearing,rear wheel bearings and the bore size in mm..so they weren't averse to the use of Metric components. If the speedo thread is Imperial the nearest size to it would be 45/64ths or 23/32nds..both of these would be odd choices though as niether are standard cycle threads. Careful checking of the pitch and thread angle is what's required..but there is only 1/2 TPI between 18mm x 1mm metric and a (non standard) 26 TPI cycle thread. This one could well be metric, although the cable attachment thread is 3/8" Cycle just to confuse matters....Ian

email (option): ian@wright52.plus.com

Ian, I'll pass on these comments and the speedo drive to John Ilott next week and see what he makes of it. Ron

email (option): ronpier@talk21.com

There have been some fairly strong comments on this 'thread', considering that the intention was to identify the size and form of the right-angle drive fixing and the only interchangeability being discussed was on the critical application of the speedo mounting nuts.

I have B.A. nuts on mine because I thought at the time that it was correct but the threads on old Jaeger cases are inevitably worn and I'm quite prepared to admit that I could be wrong.

I've got a couple of NOS drives here and they both give an OD of 18.12mm - They are of course mazac so they might have grown a little over the last sixty years.

My 'modern' thread gauge is still AWL and the old one doesn't have metric for comparison but I have to say that it doesn't look quite right for 26tpi when held against a stong light.

The rear retaining nut is a bit of an oddity - I'm coming up with 23.54mm across flats - .92" or roughly 59/64" - could it be an undersized 15/16" ? It could of course be 24mm.

It wouldn't be the only example of imperial hexagons on metric threads. KLG 18mm plugs for the 16H have 1" hexagons but I also have some with 7/8" hex. (are they aircraft plugs ?)

I rechecked and I still make it 18mm diameter, and that's with a mike not calipers and rule.
Of course, it could be that M18 X 1 is today's nearest equivalent, and the thread is a bastard used by Smiths/BSA. The alternative 37/64ths 26 TPI thread seems too obscure for there ever to have been a commercially available tap that size; we know that some decisions taken by the factories pre-war seem strange now, but surely even then they would have just gone for a common 3/4 thread?
Anyway, seems I shall have to spring for the required metric tap and hope it's satisfactory, and I'm guessing that it really will be a one-time use tool..
Thanks all.

email (option): kevin.sandford@ntlworld.com

Kev, we can't blame BSA for this one. It's the same on Nortons. The implication is that the threaded boss fixed to the brake plate is also a Smiths component. Period adverts suggest that the fork brackets were so they certainly supplied more than the basic instrument.

I gave my speedo drive to my friend and engineer John Ilott (Some of you know him). He's studied it under his 100 x micro-screen. These are the measurements he's just come back with.
23/32" diam. X 26 TPI, pitch 0385", thread angle 55 degrees (Whit)

Ron

email (option): ronpier@talk21.com

Bizarre..So it's 26 TPI but with a Whitworth thread angle not the usual Cycle thread 60 degrees. I'll have to check this out in my 'Guide to World Screw Threads' ref. book. (which my missus calls the worlds most boring book.. ).
It seems it may be a classic 'bastard thread' which would be problematical if a tap or die was required..Ian

email (option): ian@wright52.plus.com

Hi Ian,

I have that same book! Boring? Not for us!

email (option): britool51@hotmail.com

Ian, I just phoned John with your comments. He said to tell you he put the thread on his shadow graph (whatever that is??) and it's definitely 55 degrees. He also said that BSC is 55 degrees...not 60!
Please be aware this is all light-hearted and I wouldn't know either way.

Ron

email (option): ronpier@talk21.com

Hi Ron..BSC is the 'odd man out' of the major Imperial threads having a 60 degree angle...but none taken .A shadowgraph shines a light from behind the sample onto a screen, enlarging the image..this allows small items to be viewed, and measured, at a greater magnification... Robb...great bedtime reading..Ian

email (option): ian@wright52.plus.com

Ian. Just spoke to John again. Bear in mind he knows sod all about mocicles. He asks if you can check your books on the thread angles for 26 tpi Brass and Model engineers. Which is probably what he's basing his thread angles on. Ron

email (option): ronpier@talk21.com

hi,just to confuse matters,my twopenneth,someone posted recently
that the speedo bezle on a smiths chronometric is 80 x 0.9 mm
pitch,this i have checked and is indeed correct.
i dont have this type of drive unit so cant check that.

0.9 mm = 0.03543'' and 26 tpi = 0.03846''a difference of 0.003''
per pitch which should show up clearly with a thread gauge,
or even better on a shadowgraph.
cheers rick

email (option): richardholt@rocketmail.com

Hi Ron...Brass and Model Engineers threads are both 55 degrees....Ian

email (option): ian@wright52.plus.com