7S26B vs 7S26C
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Thread: 7S26B vs 7S26C

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  1. #1
    Member MrDisco99's Avatar
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    7S26B vs 7S26C

    I'm in the process of swapping the movement on my old Seiko 5 and I'm noticing some differences that I figured you guys might want to know about.

    My plan was to just swap out the old broken 7S26B and replace it with the 7S26C from my newly upgraded SKX. I was going to swap in the calendar wheels from the old watch to preserve the stock color. This turned out to be impossible.

    Some observations...

    - The teeth on the date wheel line up differently with the numbers on the B vs C.

    - The calendar retention plates are naturally different as well. Not only does the date spring line up differently, but the location of the alignment pins on the bottom calendar plate is also different. So this part cannot be exchanged either.

    - The screw holes are in slightly different spots as well so the top plate won't swap either, not that I would need to.

    - That teensy cross head screw that you need a special screwdriver for was mercifully redacted in the C revision. The plates are held down with 4 normal screws now.

    - The other calendar parts (plastic correction wheels, 24-hour wheel, intermediate wheel, day disc) do seem to be interchangeable, for what that's worth.

    - They added a bottom jewel for the mainspring barrel on the C version! I thought this only showed up on the 4R and 6R, but I guess they did it on the 7S as well. This of course means the barrel part is different. I've only ever been able to count 20 jewels in the old 7S26B despite my best efforts to find 21, so this may actually bring it up to the proper count. Maybe they got rid of one jewel from the A to the B version?


    I went ahead and ordered a new black date wheel for the C movement, which is the same as the 4R/NH/6R date disc. I get the impression the purpose of the 7S26C revision was to standardize some of those parts across the range.

    I may add pictures later if I get the mojo...
    Last edited by MrDisco99; August 10th, 2019 at 01:46.
    ap614 and Xhantos like this.
    IN ROTATION: Seiko SARB033, SKX007 (NH36 mod), SNA411, SNK809 (rebuild), SUP880, SZSJ005; Orient Bambino V3 FAC0000DD0; Casio G-Shock DW5600E; Timex Fairfield Sub-Second TW2R38000; Red Star 1963; Vostok Aмфибия 420927; Hamilton Khaki Field Auto H70455533; Tissot Le Locle T41.1.423.33; Glycine Airman 42 GMT GL0066, Combat Sub GL0088

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  2. #2
    Member ap614's Avatar
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    Re: 7S26B vs 7S26C

    Good observations MrDisco! It is my understanding based on my own experience and parts compatibility that all your notes are correct and that both the 7S26C and 7S36C are more of a bridge between the 7S26A/B and 4R/NH movements as opposed to simply a third generation 7S26.

    Transitioning from the 7S26A to the 7S26B, the only significant change was the use of the etacheron regulator, but it had a reputation of having hair spring issues getting caught up in the regulating pin. For those that have not experienced this (myself included), it proved to increase the accuracy and tighten the +/- variance over the previous A.

    Transitioning into the 7S26C, the regulator issue was addressed with a slight pin redesign and the changes you mentioned above were made, requiring a revision to the date dial in the orientation of the driving teeth.

    As a side note, the balance (with regulator pins) part number for the 7S26B was at some point superseded by the same part number for the 7S26C's. Seiko pretty much discarded the original B part due to the issues it seems to have had. Probably a good move on their part so that suppliers wouldn't accidentally furnish them as universal parts to all 7S26's.
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  3. #3
    Member MrDisco99's Avatar
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    Re: 7S26B vs 7S26C

    OK I took some photos so you can see what I'm talking about. Top is the 7S26B, bottom is the 7S26C.

    Please excuse the scratched, dirty, and broken bits of the B movement. It's been through the wars and is just a junker now.

    Here's the main plate with all the wheels installed. You can see for the most part they are pretty similar. If you're eagle eyed, though, you'll see the screw holes and one of the alignment pins are moved a bit. This will be more obvious later. Also notice the new barrel jewel at around 12:30. It looks like the barrel may have also moved which would imply some other parts may be different as well, like the train wheel bridge. Ignore the broken date corrector wheel.

    EDIT: I just realized after looking at these pictures online that it looks like the 24-hour wheel may have moved slightly clockwise. I think that's what made it look like the barrel moved, when it actually probably hasn't.

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    Here are the bottom calendar spring plates. You can see the holes for the screws and alignment pins are moved around. Also one of the springs is a little longer than the other.

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    Here are the top calendar retention plates with day spring. You can see the screw holes are in different places. The top two are closer together and the bottom two are further apart.

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    Here are the date wheels. Notice how the teeth don't line up with the numbers the same way.

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    Here are the calendar works partially reassembled. You can see the top alignment pin moved from around 12:00 to around 1:00. The top screw hole also moved closer to 12:00 and the bottom right screw moved to the other side of that alignment pin.

    Name:  IMG_1640.jpg
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Size:  1.86 MB

    And here they are mostly reassembled (minus the day wheel). You can see the bottom right screw not only moved, but is changed from that pernicious tiny cross head to a regular flat head like the others.

    Name:  IMG_1641.jpg
Views: 98
Size:  1.89 MB
    Last edited by MrDisco99; August 10th, 2019 at 17:37.
    IN ROTATION: Seiko SARB033, SKX007 (NH36 mod), SNA411, SNK809 (rebuild), SUP880, SZSJ005; Orient Bambino V3 FAC0000DD0; Casio G-Shock DW5600E; Timex Fairfield Sub-Second TW2R38000; Red Star 1963; Vostok Aмфибия 420927; Hamilton Khaki Field Auto H70455533; Tissot Le Locle T41.1.423.33; Glycine Airman 42 GMT GL0066, Combat Sub GL0088

    EX-ROTATION: Invicta Pro Diver; Rodina R005; Seiko SSB033; Timex Weekender 38, Easy Readers
    ; Vostok Aмфибия 420059

    CATCH AND RELEASE: Casio GWM5610; Seiko SKX013, SKX173; Tissot Automatics III, Le Locle Chronometre; Glycine Combat Sub GL0185

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