Smiths Chronometric S433/1/L threads

[T Batnes]

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.

[david.anderson]

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

[T Batnes]

Great, thanks. Will fit M6 stainless nuts tomorrow

[Bob.Murphy]

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 .

Bob.

[T Batnes]

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....?

[Mick D]

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

Regards Mick

[Vincent.vanGinneke]

Yep, metric they are

[bob cottam]

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

[david.anderson]

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

[roger.fellows]

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