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Smiths 12.15 movement: mushroomed balance staff – seeking advice


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I am trying to repair a recently acquired Smiths “braille” watch. It’s a lovely looking watch in almost mint condition, with a white enamelled Swiss dial in a chrome plated case and what looks like its original celluloid glass. The balance is not shock-protected. The movement number is C163791, which as far as I can gather dates it to around 1953.

After servicing, the watch runs very well dial down but immediately stops in the dial up position. The lower balance pivot appears to be slightly mushroomed at the tip and my guess is that this is scraping on the lower hole jewel. When I was taking the movement apart it took a fair bit of coaxing before the balance staff parted company with the lower jewel – I thought at the time that it might be stuck with dried up oil but I now put this down to the mushroomed pivot. Happily the jewel seems to be OK.

What to do? I don’t have a Jacot tool or a lathe, and I’m not aware of any other means of repairing the pivot. But I do have a staking set, so my plan was to attempt my first balance staff replacement. I found an online Ronda catalogue, which says that I need a Ronda 4818 (374A) for a Smiths 12.15, with an overall length of 3.74mm. I’m aware that the 12.15 comes in non-shock and shock-protected versions but the Ronda catalogue only gives one staff model and doesn’t say whether it is for one type or the other. Perhaps they are both the same length? I have measured my balance staff at approximately 3.55mm, which makes me suspicious that the Ronda staff will be too long.

I’d be very grateful for any advice on how to proceed. Am I looking at the correct replacement? If not, how do go about sourcing the correct model? Are there other manufacturers of balance staffs?

Of course, another option would be to find a donor movement, but I’m not sure if the specifications of the non-shock 12.15 models changed over time. Perhaps someone with experience of this movement could kindly advise. The final trick would be finding one with a good staff/balance!

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It looks like Ronda 4818 is the one you want. I can't find any reference to a shock protected staff.

I've never heard of a pivot becoming domed. They are hardened so should only wear down, not deform.

Do you have the means of taking a photo of it?

Changing a staff is not that difficult. It's like a lot of skills in watchmaking, once you've done it a few times it becomes easy, but it's a bit scary the first time.

There are plenty of Youtube videos showing how to do it.

 

 

Edited by mikepilk
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20 minutes ago, mikepilk said:

I've never heard of a pivot becoming domed. They are hardened so should only wear down, not deform

Perhaps he means this is wear between the pivot tip and the pivot, as though the jewel hole has worn though a section of pivot above the tip. This I've seen on one or two that i have restored, it makes the tip look vaguely mushroom shaped. 

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1 hour ago, Neverenoughwatches said:

Perhaps he means this is wear between the pivot tip and the pivot, as though the jewel hole has worn though a section of pivot above the tip. This I've seen on one or two that i have restored, it makes the tip look vaguely mushroom shaped. 

Good point Rich. My Longines I just serviced had this exact problem - years of running with no lubrication.

It definitely needs a new balance (and possibly jewels) 

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2 minutes ago, mikepilk said:

Good point Rich. My Longines I just serviced had this exact problem - years of running with no lubrication.

It definitely needs a new balance (and possibly jewels) 

What cal is it Mike, i think i have some longines staffs, i can have a look though later today to see what i have, might get lucky. If you have dimensions of it as well.

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11 minutes ago, Neverenoughwatches said:

What cal is it Mike, i think i have some longines staffs, i can have a look though later today to see what i have, might get lucky. If you have dimensions of it as well.

I didn't mean my Longines needs a new staff (I already did that), I meant the OP 🤣

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@Bluescrew I have just measured the balance staff length in both a shock protected 12.15 (38 60461E, no idea if that means anything date wise), and a non-protected 12.15 (CO10086, dated as 1951 from the inscription on the back) and can confirm that they are different lengths, with the non-protected one coming in at 3.55mm and the shock protected one at 3.74mm, so I don't think that the Ronda 4818 is going to help you.

 

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Thank you all very much for your replies.

By “mushroomed” I meant that the tip appears to have flattened and spread outwards, a bit like the shape of the top of a cold chisel after being repeatedly hit with a club hammer.

@anilv (thanks, Anil!) used this term back in 2016 in this excerpt from his post about a watch that was behaving exactly like mine, i.e. it ran well dial down but stopped dial up:

“Assuming that the cleaning was done well, this probably means something else has happened to cause the erratic running. This usually happens when the watch is dropped, resulting in..

a. Mushroomed pivots.. if the watch is dropped dial down, the pivot will mushroom at its tip. Perversely it will run ok dial down but when placed dial up, the mushroomed bit will 'hang' on the hole jewel and the watch will stop.”

I’m afraid my microscope doesn’t have a camera attachment but for what it’s worth I’m attaching a shot taken with my phone through the x20 lens. There’s little to see but I can just about discern a slight thickening of the pivot right at the tip. I’m sure if this was more than very slight the pivot would have been trapped by the hole jewel.

At first I thought that I should be able to correct what seemed like a minor problem fairly easily, but soon came to realise that I don’t have the tools to do such an intricate job properly and would doubtless just break the pivot if I tried to improvise.

So I think a pivot or balance replacement is the answer.

Now, I acknowledge that the Ronda catalogue gives number 4818 for a Smiths 12.15 but, as Marc has kindly confirmed, the staff of the non-protected movement is considerably shorter and it’s encouraging that my measurement of 3.55mm agrees with his. There is one other 12 ligne Smiths movement in the Ronda catalogue – listed as 27 RG 214 – and it’s interesting that the staff no. 2756 for this movement has a stated length of 3.56mm, just one hundredth of a mm longer than the measured staff. I can find no reference to this calibre anywhere on the internet but the other critical dimensions for staff no. 2756 are very close to no. 4818. This does make me wonder whether the 2756 might actually be the correct staff, and that I might possibly even be the proud owner of a 27 RG 214 and not a 12.15. My movement does not have any markings to confirm the calibre so I had identified it as a 12.15 purely by size (12 ligne/27mm), jewel count (15) and images on the Ranfft (RIP) database and other sources.

Are there any experts on Smiths movements out there?!

Smiths 12.15: mushroomed pivot.jpg

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