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I have a 7S26B and a 7S26C apart on my best right now. Both rotors state clearly "TWENTY-ONE JEWELS". With the two main plates side by side, it's impossible to miss the mainspring barrel arbor hole is jeweled on the C main plate, and not on the B. Logically then, there must be an extra jewel elsewhere in the B... Except... I counted it up a half dozen times, then wrote it down because I was clearly missing something... But I just can't find the 21st jewel on the 7S26B. Given the possibilities of Seiko lying or my swiss cheese brain missing something, money is on my own failings... So... What am I missing?

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3 hours ago, spectre6000 said:

I have a 7S26B and a 7S26C apart on my best right now. Both rotors state clearly "TWENTY-ONE JEWELS". With the two main plates side by side, it's impossible to miss the mainspring barrel arbor hole is jeweled on the C main plate, and not on the B. Logically then, there must be an extra jewel elsewhere in the B... Except... I counted it up a half dozen times, then wrote it down because I was clearly missing something... But I just can't find the 21st jewel on the 7S26B. Given the possibilities of Seiko lying or my swiss cheese brain missing something, money is on my own failings... So... What am I missing?

No lie. Have you counted the impulse jewel. 

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2 hours ago, jdm said:

Have you counted the impulse jewel.

Isn't the impulse jewel common to both movements?

Since the "C" has a jewelled barrel arbor hole which isn't jewelled on the "B", in order for the jewel count to be correct for both movements the "B" must have a jewelled bearing that is not jewelled on the "C".

It would be interesting to see what aspect of the "C" was improved by removing a jewel, even if it just turns out to be the cost of manufacture.

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Impulse jewel counted on both.

I've owned both since new. The only differences between the rotors are B and C after 7S26. I googled it a bit, and found others also coming up short, and haven't found an answer.

I can't promise any fancy editing or anything, but I'll see about getting some photos up this evening if I don't break myself too badly with today's activities.

Edited by spectre6000

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On 6/7/2020 at 4:47 PM, spectre6000 said:

I googled it a bit, and found others also coming up short, and haven't found an answer.

Winding reduction wheel has 2 holed jewels
Center wheel has 2 holed jewels
3rd and escape have 3 each, 2 holed but only upper side are anti-shock with cap jewels.
Pallet fork have 2, it pivots on 2 more.
Balance has 1, it pivots on (anti-shock) 4 more 
Total 21. 

There are also 23J versions, these have anti-shock for 3rd and escape on small plate.

If you want to know their P/N, check my document

https://docs.google.com/spreadsheets/d/e/2PACX-1vRN2UULQKTfKmhRStZhDdIOIQrqd6sPB-g6x2SKyQQjOvTBjG_7TQXQhAT4f1WqAX5QAPkIimi-3jqd/pubhtml


 

Edited by jdm

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7 hours ago, jdm said:

2 = Winding reduction wheel has 2 holed jewels
2 = Center wheel has 2 holed jewels
6 = 3rd and escape have 3 each, 2 holed but only upper side are anti-shock with cap jewels.
4 = Pallet fork have 2, it pivots on 2 more.
5 = Balance has 1, it pivots on (anti-shock) 4 more 
Total 21. 19 (I think you counted the "2" in your explanation of the 3rd/escape having cap jewels to get to 21)

I don't have it apart in front of me, but I think the one you're missing is the seconds wheel, which goes through the center wheel, has a jewel on the same axis as the rotor/center wheel on the underside of the threaded post for the rotor. Then the 7S26C adds the jeweled barrel arbor for 21.

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On 6/7/2020 at 10:50 AM, Marc said:

It would be interesting to see what aspect of the "C" was improved by removing a jewel, even if it just turns out to be the cost of manufacture.

Answer below.

5 hours ago, spectre6000 said:

Total 21. 19

Sorry, somehow I've killed the line about seconds  (fourth) wheel before posting, and in there lies the explanation.
In A and B version, the center wheel bridge holds two holed jewels. These are described in my document linked above: Center wheel upper hole jewel, and Fourth wheel lower hole jewel, with their respective P/N. Check picture below.

P6211391.JPG.cc51b50dabadb29c5947fe70ba654eeb.JPG

With the C version they cleverly unified these two in a single one, and the bridge got a new P/N, also pictured

P6211392.JPG.519d91eb9b8957581288bc9a89b140ec.JPG

Incidentally, also the 6R15 got identical changes when going from B to C. So to resume:

7S26A and B:
2 - Winding reduction wheel has 2 holed jewels
2 - Center wheel has 2 holed jewels
6 - 3rd and escape have 3 each, 2 holed but only upper side are anti-shock with cap jewels.
2 - 4th (seconds) have 2 holed jewels
4 - Pallet fork have 2, it pivots on 2 more.
5 - Balance has 1, it pivots on (anti-shock) 4 more 
Total 21

7S26C:
2 - Winding reduction wheel has 2 holed jewels
1 - Barrel has 1 holed jewel
2 - Center wheel has 2 holed jewels, of these 1 is shared with the 4th wheel.
6 - 3rd and escape have 3 each, 2 holed but only upper side are anti-shock with cap jewels.
1 - 4th (seconds) has 1 holed jewel,
4 - Pallet fork have 2, it pivots on 2 more.
5 - Balance has 1, it pivots on (anti-shock) 4 more 
Total 21

There are also 23J versions, these have 2 upper cap jewels for 3rd and escape wheels on small plate.

 

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So there are two jewels essentially in the same hole with (I assume) identical ODs and different IDs? Fascinating. Definitely did not notice that with them side by side.

That brings me back to my original thought process when I saw the main plate for the C having the jeweled barrel arbor: is it possible to combine parts to make the best of both worlds? It looks like you'd need the center wheel, center wheel bridge, and the seconds wheel (is it still called the fourth wheel when it's not the fourth in the train from the barrel?) from the A/B on the C... I'll have to fiddle around with it next time I'm at the bench. 

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