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Seiko 7S26A Complete Service Walkthrough

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Seiko 7S26A Complete Service




I have a good friend, a brother in Christ Jesus, who I've known for many years.  He knows I've embarked on retraining myself for a new career in Watchmaking, and seen my first two restored watches.  He told me that his old faithful Seiko 5, which he's worn everyday for 12 years, has recently had issues.




Occasionally it will advance rapidly in time (up to an hour in a few seconds) and then just keep ticking away normally.  I told him I'd be happy to take a look at it, and put it on my ACEtimer Timegrapher.  The pattern on the screen looked like a B-52 drop in Nam (stupid me forgot to take a photo), and I told him that his watch definitely needed an inspection and service.  So started my research on what the problem might be.


After reading a "Practical Watchmaking", and the many forums that I've read, I was pretty sure it was the Pallet Fork ... either very dirty or damaged stones, or a broken/damaged fork pivot.

So onward to the service...





One unusual aspect of this watch is the crown ... or lack there of, more to the point.

I suppose since this is an automatic watch, they thought it didn't need to be wound.




This watch also has a display back, so extra special care not to mark any of the plates, or damage screw heads!




The first issue you'll face when working on a 7S26A Movement, is how to get the stem out!




It isn't obvious at all, and there is a little trick. 

The crown needs to be pushed all the way in to expose the push plate (it is hidden in the other crown positions).  I took this photo once the movement was out to best illustrate where to push.




Remove the Hands, Dial and Oscillating Weight (2.0mm Screwdriver).




Gently lever up one end of the circlip and carefully work your way around.




You then should be able to raise the circlip up the length of the shaft without it pinging off.




Remove the Day Wheel and the four screws holding the Date Dial Guard.  (Use a 1.40mm Screwdriver, and this driver is good of all the screws from now on; bar one.)

NOTE ORANGE ARROW: Seiko Special Tool needed for the 0.98mm Philipshead Screw (Part Number: S-921)

I had to journey down to my nearest Seiko Distributor and grab one ... cost was AU$24.00




Here's a closer look at the troublesome screw.




Remove Date Jumper, and note that the Date Drivewheel lips over the top of the plate.




Remove all the motion work, and pull the Cannon Pinion




Remove the tension from the Mainspring.




Remove the Ratchet Wheel and the Second Reduction Wheel and Pinion. (remember the Reduction Wheel has a reverse thread)




Unscrew the Balance Cock and remove the Balance.  Also unscrew the Pellet Cock and remove the Pellet Fork.




BINGO!  Found the problem with my friends watch.  The top pivot on the fork is broken.




Easy fix with a replacement fork :)




Remove the Barrel/Train Wheel Bridge




Remove the Click, then the Barrel.  Remove the Fourth Wheel, Third Wheel and Escapement.




Unscrew the Centre Wheel Bridge and remove the Centre Wheel




Now to the Keyless Work.  Remove the Setting Lever Spring




Remove the Yoke and the Setting Lever




Pull the Stem out, and the Clutch and Intermediate Wheel will fall away.

Lastly, pull the black plastic location ring off.




... and now it's bath time!!!




I hope this has been of help to you guys.

I'll post the Assembly steps in this thread tomorrow morning.



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Nice watch. I'm wearing the same model right now.


Thanks very much for taking the time to post this. I'm looking forward to the reassembly. I have a 7s26 with a damaged pallet stone to fix.


May I ask what cleaning machine that is?



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May I ask what cleaning machine that is?




Hi Don,


Here's the link to my first watch that I ever serviced .... it has the information on the Janta Cleaning Machine.



If you're in the USA send "bobm12" a private message about getting one, he should be able to hook you up.

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This is a very easy watch to re-assemble.  In fact I'd recommend the 7S26A to be the first Japanese Movement to practice on, because it's so beautifully and logically laid out.  The only downside to this watch is that you have to buy one special tool from Seiko, the S-921 Philipshead Screwdriver; but it's not very expensive and once you've got it, you have it for life.




For oiling information and a parts list for your movement, please download the copy of the Tech Guide below:



Drop in the Clutch and Intermediate Wheel.

Then push the Stem into the shaft.




Fit the Centre Wheel, and attach the Bridge.

Then drop in the Mainspring Barrel, and refit the Click.




Refit the Keyless Work, and I have to say the Seiko design is a HUGE improvement over the AS and ST Keyless Work design ... no fiddly Setting Lever screw to worry about ... Love it!!

Once the Keyless Work is installed, install the Third Wheel, then the Fourth Wheel, and lastly the Escape Wheel.




Next, fit the Barrel/Train Wheel Bridge.

PLEASE NOTE: Be careful to check that you have pulled the Click out from underneath the bridge BEFORE you begin locating the pivots of the train.




Once all the pivots on the train are in place, and you're happy it's all moving freely, use the three longest screws to secure it down.




Install the Ratchet Wheel, and the Second Reduction Wheel.

Two things to note here:

Firstly, when screwing down the Ratchet Wheel, make sure to clear the leg of the Pawl Lever, as it has a nasty habit of wanting to slip under the screw.

Secondly, as you screw down the Second Reduction Wheel (Reverse thread remember!), keep adjusting the Pawl Lever arms so they rest on either side of the wheel.




Next, fit the Pallet and Pallet Cock, and then the Balance.




This particular movement has the DiaShock system for the IncaBloc. 




To remove and refit these for oiling I suggest you make a simple tool from a plastic covered Paperclip.

Straighten out the Paperclip and cut it to a comfortable length to work with in your fingers.  Then pull the plastic cover down about 4-5mm ... as pictured below:


Use this like you would Pegwood, over the top of the DiaShock Spring, as you use your tweezers to turn the spring and remove/refit it.  The plastic cover will conform to the shape of the spring and hold it securely without damaging it ... It's works a treat!

PS. Sorry I couldn't get a photo of me removing/refitting the spring with this little tool; but I don't have someone here to do the camera work, and only have two arms.





Next, flip the movement over, and reinstall the plastic Location Ring.




Install the Cannon Pinion, the black plastic Date Driving Wheel, then the Motion Work, and lastly the Calendar Ring.




Install the Date Jumper Plate

Install the white plastic Date-date Corrector Wheel and it's Intermediate Wheel

Note: Remember from disassembly that the black plastic Date Driving Wheel needs to lip over this plate.




Install the Date Dial Guard




Place the Day Disk on the shaft and push the Circlip down the shaft after that and push down until it locates in the Circlip Groove.




Lastly, reinstall the Oscillating Weight, Dial, and Hands.

Case back up and your done!




I really enjoyed working on this movement, and I thank my Heavenly Father for the skills and ability to be able to do this type of work ... amen!





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Very nice Lawson! That sums it up nicely. Pity you couldn't illustrate the paper clip trick! I suppose I'll be trying that one too.


I got prices from CousinsUK and with shipping and all, it is cheaper than buying it (surprise!) here so I got both the balance and the fork on order.


By the way, I've just finished rebuilding another 7S36 and I got it now on my wrist. Here it is next to its factory new lesser brother (7S26):




Regretfully the one I finished didn't come out well in this pic. (the gold color one) I got a crappy sansung cell! The band and case is normally where I enclose these movements for testing purposes. I'll post another picture later on (in another thread not to hijack this nice one).


The Seiko special screw driver, although I also bought one ($10 + s/h), can be fashioned by taking a .60 or .50 screwdriver and filing the sides at an angle (so as to make it pointy). Of course, you never finish "sharpening" it but making a narrower tip which angles away toward the shaft. That way a half philip point screwdriver is fashioned to work on that screw. I never did it that way as I said but some people have...I believe I don't even use moebius oils on Seikos since I was able to source the recommended ones (S4 and S6). Exception to be made with 9010 (S-A in the chart), 931 for the pallet jewels (Seiko recommends 9010) and I treat the main spring with 8200 when I disassemble and clean the barrel.


As a side note, and just for the record and completenes here, the only difference between the 7S26 and the 7S36 is an extra 2 jewel caps (not much of a difference). All parts are interchangeable between the two models and you can even make a 7S26 into a 7S36 by adding that little bridge containing the 2 jewels since the plate is ready and threaded to accept it in the lower jewel count movement. It is said that the 7S36 is a little better finished but apart from the advertisement edged on different places I think it is all cosmetic.


Thank you for such nice, detailed illustration of the Seiko 7S28/7S36! Great Job!

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"NOTE ORANGE ARROW: Seiko Special Tool needed for the 0.98mm Philipshead Screw (Part Number: S-921)"


Is that special phillips head screw highlighted on your picture of the JIS type by any chance, or something proprietary?


I tried to find that part number for the screwdriver but was only able to find more posted questions about it.


I picked up a set of these JIS phillips screwdrivers, not sure if they would be commonly needed and if so, I'm glad I did.

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It's so tiny it's hard to tell, but looks more JIS, but definitely proprietary due to the size ... it's the smallest philipshead I've ever seen by far.


I couldn't find any close up pictures of the tip but the part number you supplied for the actual screwdriver is mentioned variously elsewhere.


The smallest is 1.2mm in the JIS set but that screw head looks much smaller.


Thanks for sharing Lawson.

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Rosso, I got mine from esslinger. I also got the Seiko silicon grease to make crowns and backs more water resistant. As I pointed out, the Philips screwdriver can be fashioned from a small .5 or .6 mini screwdriver by filing, but I don't think it is worth it since the real thing is available.

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Rosso, I got mine from esslinger. I also got the Seiko silicon grease to make crowns and backs more water resistant. As I pointed out, the Philips screwdriver can be fashioned from a small .5 or .6 mini screwdriver by filing, but I don't think it is worth it since the real thing is available.

Thanks Robert.  I just placed an order with Esslinger too to see what delivery and availability would be like here to Canada so its good to know they have it.  I see what you mean I think, about taking the file to the straight blade screwdriver (?), I assumed that was what you were referring to and not modifying a regular small phillips head screwdriver?  Bear with me, I'm new :)

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I found the proper special tools needed to remove/install the keepers on the Diashock Keepers, and thought it poignant to post it here also, so that it's all in one spot for working on this movement.




Here are the Tools




First is the 2mm Bergeon Barrel Arbor Holder

Cheapest found online here:





Attachments are called KIF Shockspring Tool

Available at Cousins in the UK





Hope this helps with the frustrations of these little fellows.

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Thanks. Even the calendar wheel?

Don't put anything with with numbers or paint into the ultrasonic bath, it could destroy the finish. A gentle wash with soapy water using a cotton bud then a good rinse, if cleaning is required.

Edited by Geo
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