Vintage Workshop
Services for Brough Superior motorcycles and their contemporaries

A tricky repair to a SMITHS chronometric speedo head

Recently, I bought a Smiths speedo head on ebay for my project. Or rather, one to go on my MX 80, which is still bearing an instrument with a wrong calibration constant, reading about 8% high.

I just love these little mechanical marvels. And I had, so far, not come across one that needed more than cleaning and oiling and some cosmetic attention.

Unfortunately this one proved to be a lemon.
A peep into the bulb holder hole told me there was a problem with the balance wheel.

Opening the case took me the best part of an hour, as the bezel was horribly tight, and there was a dent in it, which went right through into the thread on the shell.

With the movement on the table, the problem was obvious:
The main plates of the movement had come a bit apart (due to loose screws), and the balance wheel had dropped out.

Now that would not have been so bad, but either somebody had tried to put it back by poking into the bulb holder aperture with a screwdriver, or the whole unit has been dropped hard.

Whatever the reason, the balance staff was bent on one end and broken on the other.

I took the balance to pieces and made a drawing.
Mind you, these are millimetres...

A pretty small bit to manufacture, but just imagine the dimensions of the corresponding component of a wristwatch!!

I did not fancy turning such a small bit on my lathe. And besides, I would like the pivots to be hardened.

So I decided to grind it from a 1.5mm drill shank on my lovely little Deckel S0 tool grinder.

This is a pic from grinding one of the pivots...

... and here, the other one and the tapered portion.

I found the diameter of the taper needs to be accurate to 0.01mm, otherwise the pressed-on balance wheel and the escapement bit will not go on or it will be too loose.

This is the finished balance staff and the bits to go on it.

An this is the assembled balance unit.

I just wonder if the very hard HSS material will be too brittle.
Experience will tell.....

I put everything back together and the movement went just fine!

Altogether I spent a lot of time, but I learnt a few lessons. And I got into some websites dealing with watch and clock repair, and I can only say I have a deep respect for these people!

see e.g. or
just have a look!

- sorry pic missing
and speedo already completely assembled-

Any kind of feedback to is appreciated
(sorry, this is not a clickable 'mailto:' hyperlink. If you want to write me, please type my address in your mailer. )

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