Master Watchmaker, Clockmaker and Jeweller (since 1981)
omega panerai iwc patek and other fine watches by Nicholas Hacko Sydney
Suite 403, Level 4, Culwulla Chambers
67 Castlereagh St. Sydney 2000 NSW Australia
Phone: (02) 9232 0500     Fax: 02 9233 2273

Do It Yourself Project: Seiko 7S26 for Novice Horologists

Chapter 9: The oscillator and the gear train

In this chapter we will continue with the disassembly of the remaining units, starting with components in the power source unit, then the oscillator and then we will disassemble the counter or train wheels. In addition, the last step is to pull apart remaining components in the time and calendar setting assembly.

There are a couple of potential 'booby traps' so please read the entire chapter before proceeding with disassembly.

We have also reached the stage where we will have to remove bridges or plates which hold the components in place. It is important to understand that once screws are removed, the bridge is still secured in place because it's held by alignment pins. Therefore most bridges must be 'lifted' from the main plate 2-3mm before being removed. Placing a screwdriver at that exact point is necessary in order to avoid any potential damage to the components underneath the bridge.

Another important thing is to hold the components with tweezers at a certain point (for example it is fine to hold the large pinion directly with tweezers, but that would be inappropriate for smaller ones). If in doubt, refer to photo.

The most sensitive of all components is the hair spring which is attached at one end to the balance cock and another end to the balance wheel.

Seiko Automatic 7S26 hair spring> <p><img src=

Seiko Watch 7S26 Movement>
<p><img src=

And here are couple of examples of inappropriate handling:

Seiko automatic movement

The balance wheel assembly should be placed / rest in 'cock down' position, not the other way around!

Seiko automatic watch movement 7s26

Seiko 7S26 Automatic

Another way of mishandling is to remove the assembly without paying attention to the components which may be obstructing the hair spring or the balance wheel. In other words, the hair spring or the balance wheel could be partially "tucked" under by a nearby component like a bridge or wheel.

In the case of Seiko 7S26, the hair spring itself is not obstructed, however the balance wheel is! It is sits partially under the 4th wheel and train bridge.

Seiko watch

Pulling the cock directly vertically during assembly will result in deformation of the hair spring - you will be pulling the cock up, but the balance wheel will be obstructed so the spring will stretch beyond the point of elasticity. Again, pay attention to the images below and you'll be fine.

We will talk about the balance wheel assembly in more detail later on.

Turning the power "off"

Before we can proceed further with disassembly, we need to unwind or uncoil the main spring. Now, if you have commenced disassembly more than 2 days ago, your watch has stopped and the spring is already unwound. However it is important to do the following steps regardless.

As we already know, the auto winding unit winds the spring by turning the ratchet wheel clockwise. We can wind the clock 'manually' too, by turning the ratchet wheel screw clockwise. It takes 8 rotations ( 8 x 360 degrees) to wind the spring fully. However we will wind it just two turns, to get the feeling of winding action and spring tension. So turn the ratchet wheel CW, two full rotation.

Seiko 7S26 ratchet wheel

The unwinding of the main spring is a three-step process.

    1. Turn the ratchet wheel for a few degrees (few clicks) and hold the tension
    2. Unlock the ratchet spring by pushing it gently towards yourself until it disengages from the ratchet wheel and HOLD it disengaged with tweezers.

    Seiko 7S26 Movement - turning the ratchet wheel

    3. The spring is now trying to overcome your holding pressure by turning the screwdriver anti-clockwise. Slowly release the grip on the screwdriver body and let the spring unwind. Attention: Do not let the screwdriver turn instantly - instead control the unwinding action with the "right amount of grip."

    Seiko Automatic Movement

TO DO: Repeat the manual winding action and unwinding process as described before 6 to 8 times starting with two turns, then 3, etc. until you can confidently control release of 8 full turns (completely wound / unwound ) main spring.

Unscrew ratchet wheel screw.

Automatic Seiko Watch 7S26

Remove ratchet wheel.

Seiko Auto 7S26

Disassembly of balance complete

The balance cock or balance bridge is the main body of the bal. wheel assembly and it carries all components of the assembly - the wheel, hair spring, upper jewel / shockproof assembly, both regulating levers and studs.

Seiko 7S26 balance bridge><p>

It is <u>absolutely crucial</u> that regulating levers are not moved. The best is to leave them 
as they are! At this stage our aim is only to detach the balance cock from the main (base) plate.

<p><img src=

Remove balance cock screw.

Seiko Automatic Watch DIY Repair

Note that the cock is held in position with two alignment studs. Place a screwdriver underneath the cock as per photo.

Seiko DIY Assembly 7S26

Twist the screwdriver to lift the cock up just enough to detach it from studs.

Seiko DIY Assembly Tutorial

Now hold the cock with tweezers.

Seiko Automatic Watch tutorial

Lift the cock up for another 3-5mm.

Seiko 7S26 DIY Repair

While holding the cock 5-8mm above the main plate, rotate it towards yourself (CCW) then back (CW) until the balance wheel is free form the lower jewel. NOTE: Do not lift the cock too high because the balance wheel is obstructed by other components!

Seiko DIY Automatic Watch assembly and disassembly>
Once the wheel is out of the jewel, bring the cock towards yourself a bit to clear it from the bridge and 4th wheel.
<p><img src=

Finally, remove the cock and entire assembly by lifting it up, towards yourself.

Automatic Seiko Watch DIY Repair

Rest the assembly as per photo.

Automatic Seiko DIY

Automatic Seiko 7S26 Assembly DIY

The next step is disassembly of pallet fork. Remove two bridge screws.

Seiko Automatic 7S26 DIY

Lift the bridge as per photo.

DIY Repair of Seiko Automatic Watch

Seiko 7S26 movement DIY

Lift the pallets form the lower jewel, up. Hold as per photo.

Seiko 7S26 pallets>
<p><img src=

Components is oscillator unit (minus the escape wheel).

Seiko DIY Automatic Watch

Next step is disassembly of the 'counter' unit - the main train wheels, including the mainspring barrel.

Start with unscrewing 3 bridge screws.

Unscrewing Seiko Automatic watch bridge screws

Position the screwdriver as shown and lift the bridge up, by 5mm only.

Seiko 7S26 bridge

If necessary, lift the bridge from the opposite side as well.

Seiko 7S26 DIY assembly and disassembly

Automatic Seiko 7S26 DIY repair

Bridge with attached auto reduction wheel and pawl. To be disassembled later.

Seiko 7S26 reduction wheel and pawl level

Mainspring barrel, click and train wheels.

Seiko Automatic Watch - mainspring barrel, click and train wheels

Note where the click spring is positioned. Remove it.

Seiko DIY Automatic Watch Repair

Remove the wheels in the following order, starting with the fourth wheel.

Seiko Automatic Watch 7S26

Third wheel

Automatic Seiko Watch - third wheel


Seiko Watch - barrel

Escape wheel.

Seiko Automatic watch - escape wheel

The centre wheel is held by the centre wheel bridge. Unscrew the bridge screw, position screwdriver close to stud and lift the bridge up.

Seiko 7S26 Movement

Seiko Automatic Movement

Automatic Seiko Watch 7S26 Movement

Disassemble auto winding components attached to the bridge. Remove reduction wheel circlip (Seiko calls it "the holder")

Seiko Automatic Watch Disassembly

Seiko DIY Watch Disassembly

Automatic Seiko Watch DIY Disassembly tutorial

Seiko Automatic 7S26

Photo showing disassembled components

Disassembled Seiko 7S26 components

Components grouped in units.

Disassembly tutorial of Seiko Automatic 7S26

Finally we will disassemble remaining components in the time and calendar setting: the stem, yoke, yoke spring, setting lever and setting wheels.

Disassembly of time and calendar in Seiko 7S26

Unscrew the yoke spring screws and yoke spring.

Unscrewing yoke spring of Seiko 7S26

The yoke is held under the tension. Hold as shown on photo and lift the tail.

Seiko DIY Automatic Watch

Tutorial Seiko 7S26 assembly and disassembly

Seiko 7S26 assembly and disassembly tutorial

Remove the setting lever. Pull out the stem. This action will free the clutch wheel and setting wheel.

Assembly and disassembly of Seiko Automatic Watch

Automatic Seiko Watch 7S26 Movement

Automatic watch movement Seiko 7S26

Seiko 7S26 DIY Repair

Seiko watch 7S26

This completes the disassembly of Seiko 7S26 movement. Photo below showing components in Phase 9.

Components of Seiko 7S26 movement

TO DO: send me an email on to confirm that you have successfully completed disassembly. Remember: Subject line must start with DIY.

Previous Chapter | Home | Next Chapter

Mailing list: SEIKO 7S26 DIY Mailing list home page

To join please click above link or send a blank e-mail to

The mailing list will serve to allow better communication between 'students' and allow the rest to follow the progress of our project. The group is not moderated so please focus posts on topics related to the DIY project: assembly tips, general observations, help requests, tools and parts exchange, project photos and DIY comments.

There is only ONE RULE: please sign all posts with your full name and location :-)