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It is easy to develop a misapprehension, when first dipping one’s toe into the pool of what a particular watch company’s vintage back catalogue has to offer, that every watch is fitted with one or other minor variation of the same movement calibre. I plead guilty to forming just such an impression during my formative years as a vintage Seiko obsessive. It seemed to me at the time that pretty much every watch worthy of consideration was fitted with some variant of the Seiko 61 or 62 series movement, with the lesser fare served by a movement caliber beginning with a 7. The honorable exception to the rule was the Bell-Matic with its innovative 4005/6. It is all too easy to categorize as a result every Seiko as a non-hacking auto lacking a manual winding facility and further to conclude that the middle of the road staple Seiko movement is placed a notch or two below the lofty sophistication of say an ETA 2472, only countering in its ability to run for decades without maintenance.

Until relatively recently, my perspective on the Seiko movement landscape of the 1960’s was of a foundation defined by the venerable 62 series, fitted in various forms to all and sundry from aspirational to royalty, and subsequently replaced by the 61 series, also fitted hither and thither to pretty much all levels within the Seiko model hierarchy. However, that perspective fails rather to take account of a number of sophisticated, beautifully designed calibres sitting left of field, one notable example being the svelt 83 series automatic, which made its entrance as early as 1963.

The base 830, an automatic without calendar function was fitted to the Seikomatic Slim released in August 1963 and marketed for its slender profile.

Seikomatic slim vs Seikomatic

The sleek Seikomatic 830 (left) compared to the heavier-set Seikomatic Weekdater 400 (right)

At 28.6 mm diameter, the 830 calibre was a little wider than the 603 fitted to the first Seikomatic released a few years earlier, but at 3.8 mm thick it was a full 1 mm thinner than the 603.  This allowed for a notably shallower case back and consequently for a watch that sat lower on the wrist. This lack of height was achieved by incorporating the autowinding mechanism into the main architecture of the movement, concealed beneath its own bridge at the same level as the train wheels. A date complication was introduced in June 1965 in the 840/8305B, resulting in an increase in height of 1mm but still 1 mm thinner than the equivalent dated 62 series movements. The C variant of the 8305 followed a few months later, now also featuring a hand winding facility. All variations of this movement were highly jeweled, every one of which serving a purpose.

This brings us nicely to our subject, an 8305-8030 Seikomatic-R, dating from October 1966.

Seikomatic R 8305

This particular model was released as part of a drive by Seiko to expand its Seikomatic range into the business market. The somewhat more conservative styling, aping to an extent the ‘60’s Omega dress watch aesthetic, aligned with that ambition. The incorporation of a manual wind capability in the 8305C fitted to this model also resulted in the relocation of the crown from the 4 o’clock position in the earlier 830 and 840 models, to the more accessible 3 o’clock.

The age of my example is hinted at largely through the scuffed condition of the crystal and the presence of plenty of case grot but otherwise it appears to be remarkably unscathed by the 50 years or so of its life. One aspect of the watch not apparent from the view from the front is that, in common with some of the contemporary dressier Omega Seamaster watches, this particular model features a one-piece mid case.

Seiko 8305 one piece case

The relative lack of wear to the case is evidenced by the presence still of the crisply embossed dolphin, there to trumpet the water proof credentials of the watch. The advantages offered to water resistance in the unibody case are offset to an extent by the resulting obstacle it presents to the gaining of entry to the innards. The first step is to remove the bezel surrounding the acrylic crystal, in the process revealing the inevitable build up of dirt and some corrosion beneath.

Seiko 8305 bezel

The crystal is compression-fitted to the case, aided by the tension ring sitting around the inner circumference of the crystal. Its removal requires a tool to compress the crystal so that it releases its grip on the case sufficiently to aid removal. I do have a Seiko S-14 one piece case opener but the set I purchased is missing the correct crystal tightening ring for this watch and so I have to use my conventional crystal lift to extract the crystal.

Seiko 8305 crystal lift

Seiko 8305 crystal removed

The next problem is how to remove the crown and thence the movement when the setting lever pin is located to the rear of the movement. Closer inspection of the channel around the dial adjacent to the crown reveals a helpfully dimpled tab. In the spirit of Alice in Wonderland, I take up the implied invitation to ‘press me’.

Stem release tab

8305 stem released


With the stem released we still need to rotate the case spring around so that the inner tabs align with strategically located gaps at the edge of the movement main plate.

Seiko 8305 case spring

Finally, we can incline the case and carefully ease the movement out. With the movement safely sitting on a movement holder, the lovely condition of the dial, markers and hands is easier to appreciate.

Seiko 8305 dial

The convex dial curves down to meet the edge of movement main plate, leaving no room for a dial spacer. With the hands removed, the dial comes next, once the recessed screws gripping the dial feet have been loosened.

Seiko 8305 dial screw

The view provided of the calendar side yields no surprises; a conventional layout not dissimilar to that used on the 62 series.

Seiko 8305 calendar

I’ll proceed from this point more or less in the order suggested by the Seiko technical guide, starting conventionally with the calendar side, stripping it down as far as the keyless works.

8305 calendar disassembly

Once we’ve got to this point, we can flip the movement over and survey it from the rear, a view not offered until now because of the absence of a removable case back.

Seiko 8305 winder side

We can get a better view of the layout of the movement on this side by removing the rotor (oscillating weight).  In particular, this allows us to see where the automatic winding mechanism has been secreted.

8305 automatic winding

The disassembly of the automatic winding mechanism proceeds as follows: first remove the locking wheel bridge (top left and right, below), followed by the roller locking wheel (the multi-tiered wheel) and the first reduction wheel; next, remove the second reverser idler before levering off the first reverser idler which is secured to its post by a snap spring.

8305 autowinder disassembly

The transmission route provided by the rotation of the oscillating weight is shown in the figure below, taken from the movement technical guide:

Autowinder technical

The purpose of the roller locking wheel is to ensure that regardless of the direction of rotation of the oscillating weight, the ratchet wheel which winds the mainspring always rotates in the same direction (anticlockwise when viewed from above).

Roller rocking wheel

Next we turn our attention to the crown wheel ring (there to provide the hand winding facility) which is secured in place by two neat little polished screws. The ratchet click and train wheel bridge follow.

8305C bridge

In contrast to the conventions adopted in Seiko’s more mainstream movement designs, the sweep second pinion sits separately on top of a friction spring and is driven by the third wheel. You may also have noticed from this sequence of photos that the barrel arbor is jeweled along with every other pivot bearing, two of which, also benefiting from Diafix settings. This is a movement whose generous jewel count does not flatter to deceive. I know I keep going on about this, but I find it slightly perplexing and vexing that Seiko can secrete such a beautifully engineered movement in a relatively low key and conservative watch whilst playing silly marketing games with jewel counts in more mainstream upper market watches to disguise the more homespun origins of the movements fitted to those models. All a bit odd but an interesting perspective on the extent to which marketing considerations drove engineering decisions at the time.

Removing the train wheels, ratchet wheel and barrel allows access to the centre wheel bridge, whose secondary role is to provide a platform for the first and second reverser idlers in the auto-winding mechanism.

8305 centre wheel bridge

As we approach the half way point, it is worth pausing first to note how the dimpled tab sits in relation to the setting lever, there to aid removal of the stem from the dial side of the movement.

8305 setting lever

And just because I like the image, two Diafix springs leaning nonchalantly to one side, allowing access to the two cap jewels.

Diafix settings

Prior to cleaning the movement, I took the opportunity to see how the movement is secured in the case by the case spring.  With all the fixtures and fittings removed from the mainplate, we have an unobstructed view of how the movement sits in the case and how the two tabs on the case ring prevent the movement from jiggling about. The outer circumference of the mainplate sits on a ledge around the case aperture, allowing room for the oscillating weight to rotate clear of the rear of the case. The case spring then sits in its own groove above the ledge with two tabs pressing down on the upper edge of the movement edge.

8305 case spring

In order to remove the movement, you have to rotate the case spring anticlockwise until the two tabs align with two gaps, one at the 3 o’clock position and one at about 8.30.

8305 case spring released

After a thorough clean, the constituent parts are ready for reassembly but an inspection of the bearings on the mainplate reveals a chipped jewel serving the roller rocking wheel.

Roller locking wheel jewel

Were it not for the fact that I have recently acquired a Seitz jeweling tool, I would probably have chosen to ignore it and hope for the best, the jewel in question not impacting upon the running of the watch but on the auto winding mechanism.

Seitz jewelling tool

However, the chipped jewel provided me with an excuse to try out the new tool.  Removing the damaged jewel requires selection of an appropriately sized pusher and an anvil that sits on the reverse side holding the movement flat and providing enough clearance to allow the emergence of the jewel as it is pressed out of its setting.

Seitz pusher and anvil

In this case, the landscape on both sides of the mainplate was obstructive and in the end I had to press it out from the oil reservoir side with barely enough clearance on the upper side of the movement to allow the anvil to sit flat.

Pressing out the jewel

Here’s the damaged jewel (left) sitting next to its new replacement (right):

Seiko pivot jewels

And here’s the new jewel pressed back into place, viewed from both sides:

Replacement jewel

You may be able to appreciate the difficulty presented by the movement architecture in finding an anvil of the correct diameter to sit flat whilst also having a hole large enough to accept the outgoing jewel. Before pressing on, I checked the end shake by test fitting the roller locking wheel and its bridge.

End shake check

Next up, Diafix settings and mainspring. Mastering the dismantling, cleaning, refitting and oiling of Diafix jewels has for me been one of the more challenging elements of getting to the point where I feel I kind of know what I am doing. The simple business of realizing that it is best not to remove the springs altogether is a right of passage well known by many a vintage watch dabbler. It is also I think fairly common to attempt to oil the cap jewel prior to reinstalling it but this is almost guaranteed to result in the oil spreading all over the place rather than sitting in a neat circle at the centre of the jewel. The way to oil these settings is to refit the cap jewel first and then to oil the setting from the other side using an automatic oiler. I use a Bergeon 7718.

Bergeon 7718

The proof of the pudding is in the eating. Below left, the two mainplate Diafix settings with cap jewels refitted dry. And to the right, both now oiled with the Bergeon oiler. A perfect quantity of oil deposited dead centre.

Diafix oiling

The next bit that has in the past ended in mild calamity is the removal, cleaning and refitting of the mainspring, these days achieved usually without disaster.


We are now ready to reassemble the movement, starting dial side with the keyless works,

Keyless works

followed by the centre wheel, its bridge, the sweep second pinion friction spring, second reduction wheel and barrel.

Barrel and centre wheel bridge

The trains wheels go in next followed by their bridge. You will notice that the top of the sweep second pinion sits in a jeweled bearing hidden in the underside of the bridge.

8305 train wheels

8305 trains wheel bridge

It’s all down hill from here. With the train wheel bridge and balance fitted, we can wind in a bit of power and check that the movement is running satisfactorily before proceeding further.

Seikok 8305C bridge

Getting the automatic winding bridge seated proves to be a bit of a fiddle but we get there in the end.

Seiko 8305C

The final part of the puzzle on this side is the winding weight. With the mechanism itself located within the movement proper, the rear of the rotor is free from the magic lever and associated parts and all we are left with is an elegant gear and bearing, the teeth of which mesh with the first reverser idler gear.

8305 rotor

Seiko 8305C rotor

The calendar parts are straightforward and in a jiffy we are ready to refit the dial and hands,

Seiko 8305 dial and hands

and re-case the movement.

8305 recased

The crystal proves to hold up proceedings for a week or so. I thought I might be able to get away with a generic 31.5mm tension ring equipped acrylic but these turn out to have tension rings whose diameters are too large resulting in a slight gap between the edge of the dial and the inner edge of the tension ring. There being no UK suppliers of the correct old stock part to which I have access, instead I manage to source a correct crystal from a seller in Germany. It arrives within a few days and tops things off splendidly.

Seiko 8305 crystal

I wasn’t going to remark on the timekeeping out of the box other than to note that the amplitude on full wind is hitting 309 degrees. And that is with its original mainspring. Not bad for a 50 year old. One concluding shot on a nice new croc.

Seikomatic R 8305C

* Eagle-eyed readers may spot that a bit of poetic license has been used here in the sequencing of these two photographs.

Acknowledgements: My research efforts have been greatly helped by the wealth of information available at the fantastic Seikomatic resource at  http://matic6246.web.fc2.com/