Smiths Chronometric S433/1/L threads

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T Batnes
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Smiths Chronometric S433/1/L threads

Postby T Batnes » Sat Jan 04, 2020 10:27 pm

The threads on the speedo fastening pin bolts, what size are they? I tried with 1/4" BSC nuts, they thread on but are a loose fit.
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Re: Smiths Chronometric S433/1/L threads

Postby david.anderson » Sun Jan 05, 2020 2:03 am

Torgir
they are 0 ba thread so 6mm metric will do the job just fine if you cannot find 0 ba (just a different thread pitch but the same dia and tpi)
David

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Re: Smiths Chronometric S433/1/L threads

Postby T Batnes » Sun Jan 05, 2020 2:30 am

Great, thanks. Will fit M6 stainless nuts tomorrow 8-)
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Re: Smiths Chronometric S433/1/L threads

Postby Bob.Murphy » Sun Jan 05, 2020 10:43 am

david.anderson wrote:Torgir
they are 0 ba thread so 6mm metric will do the job just fine


Aaaargh NO - BA threads are cut at 47.5 degrees, Metric at 60 degrees - you will damage both threads if you force them together.

0 BA has a 1.0mm pitch (25.4 TPI), the same as a 6.0mm standard metric thread, but they are not compatible.

That's a bodge too far in my book :roll: .

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Re: Smiths Chronometric S433/1/L threads

Postby T Batnes » Sun Jan 05, 2020 11:47 am

I'll do a rummage to see if I can source a pair of 0BA nuts then, but a bloke on the FB group mentioned it was M6 cause the Smiths speedometers originated from the Jaeger instruments....?
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Re: Smiths Chronometric S433/1/L threads

Postby Mick D » Sun Jan 05, 2020 11:50 am

Bob.Murphy wrote:That's a bodge too far in my book .


Me too - but I'm not adverse to running a 0BA tap down an M6 nut as a quick fix ;)

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Re: Smiths Chronometric S433/1/L threads

Postby Vincent.vanGinneke » Sun Jan 05, 2020 12:02 pm

Yep, metric they are

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Re: Smiths Chronometric S433/1/L threads

Postby bob cottam » Sun Jan 05, 2020 2:35 pm

Surely the clue is in the name: ChronoMETRIC....

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Re: Smiths Chronometric S433/1/L threads

Postby david.anderson » Mon Jan 06, 2020 9:30 pm

Bob
The substitution of 0ba and m6 is not a ” bodge” in a low torque situation. This substitution, along with using a bolt or nut with 1 or 2tpi difference has been taught in our TAFE colleges as suitable practice for low torque situations that are subject to vibration for over 50 years. The practice is most commonly used by gunsmiths. About 40 years ago I had the telescopic site mounts come repeatedly lose on one of my rifles. I mentioned that to the gunsmith at Western Firearms in Hurstville. He rectified the problem by using screws that were 2tpi different to the thread that had been tapped in the action. I have subsequently spoken to other gunsmiths about substitute thread forms/pitches or tpi that is 1 or 2 threads different and the practice is still common and preferred over Loctite. The substitution of 0ba and m6 was first mentioned to me by world renowned gunsmith/target barrel manufacturer Denis Tobler. (Denis has a magnificent collection of vintage motorcycles ranging from a 1906 Triumph through to a 56 square 4 outfit).
I would not however substitute thread forms in any application that required a high torque figure.
Vincent, are you certain that the chronometric speedo has metric threads. BA is actually a metric thread system devised by the British for small screws used in components such as speedos. https://britishfasteners.com/threads/
The other small screws in the chronometric appear BA? to me(although I have not measured them).
David
Last edited by david.anderson on Mon Jan 06, 2020 10:25 pm, edited 1 time in total.

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Re: Smiths Chronometric S433/1/L threads

Postby roger.fellows » Mon Jan 06, 2020 10:02 pm

'metric' in that situation probably refers to measurement or measuring rather than the system name.


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