As the 1960’s rolled into the 1970’s and the quartz juggernaut started to power its way through the foundations of the established watch industry, it is curious that companies such as Seiko continued to design, develop and release brand new mechanical watch movements. In its death throes, the traditional arm of the watch industry continued to resist the inevitable, seemingly unaware of the dastardly deeds being committed in the interests of the white heat of technological development. Of course it is easy, with the benefit of hindsight, to view this as one hand not knowing what the other was up to, but the reality was probably that the different branches of a company such as Seiko were simply getting on with their jobs and continuing to do so until the inevitable influence of the market started to shape future priorities.

In the case of Seiko, there was the additional competitive driver of the two main watchmaking divisions: Suwa in Nagano prefecture; Daini in Tokyo. We have explored elsewhere the parallel development of watch lines emerging from these two branches and the different approaches they took in either refining and developing existing movements or developing fresh designs from scratch. This competitive edge saw the emergence of such engineering treats as the slim-line 83 series movement, dating from about 1963, featuring an integrated automatic winding mechanism; the fiendishly complex 51 series fitted to the P-series Seikomatics and Press-matics from 1967; the high-beat 36ooo bph 45 series, featuring instant date change and powering hand-wind King and Grand Seikos from 1968 to 1974.

Sitting between the low-beat 83 and 51 series and the very high beat 45 series, we had the 56 series, acting both as the mainstay Lordmatic movement in lower beat, 21600 bph form as well as the 28,800 bph high beat automatic in King and Grand Seiko models. The 56 series was the creation of the Suwa division and arguably its crowning glory (notwithstanding a number of design frailties described herein).

The high beat 56 series is a 25.6 mm diameter movement with a maximum thickness of 4.5 mm. It achieved this relatively low profile as a consequence of its integrated auto-winding mechanism. The movement features quickset date (and day) calendar, hand-winding and seconds hacking. However, it lacks the instant calendar changeover of the 51 and 45 series, relying instead on gradual calendar change driven by a traditional date driving wheel.

The Suwa division 56 series was introduced in 1968 and sustained in high-beat form until 1975 and in low-beat form until as late as 1978. The Daini division had, to this point, arguably produced the more ground-breaking designs, and not wanting to be seen to be resting on its laurels, set about designing its own smaller high-beat automatic calibre as a direct competitor to the Suwa 56 series. This new calibre was the 52 series, introduced initially as the 5206 in 1970 and fitted to a number of different Lordmatic designs. A year later, chronometer-spec 5245 and 5246 movements appeared in a small number of King Seiko Special Chronometers, the best known of which the 5245-6000 and 5246-6000, featuring a monocoque case design. It is the former model that forms the basis of the current entry.

As you should be able to make out from the case back photo above, this watch dates from December 1971. You should also be able to appreciate that this is a single-piece case with access to the movement through the front (as indicated by the inscription on the case back).

First impressions are very good. The case is in very clean, original condition, retaining all of its sharp lines. The dial and hands are largely blemish-free, although the hands are covered in some sort of liquid droplet drying marks. The only real signs of wear appear on the glass, which is decorated with a myriad of scratches and the bezel which has born the brunt of many of the interactions of watch with solid surfaces over its lifetime.

Other than its overall condition, the main impression for me is that this is a comparatively small watch, certainly by the standards of most of the other watches in my collection. At 35mm diameter it is a full 1mm smaller than, for example, a Rolex Datejust of the period and a full 2 mm smaller than the competitor King Seiko 5626-7000.

This being a single-body case and a King Seiko, it is not surprising to see that this watch also features a small aperture between the lugs to provide access to the fine adjuster screw, allowing regulation of the movement in situ.

You can see too in the photo above that I’ve started the process of levering the bezel away from the case. In common with many of the other contemporary King Seikos of this period, the crystal is a V-type frame-bonded Hardlex, sitting within a complex profiled rubber gasket.

The crystal and gasket lift away to reveal the means to release the movement from the case.

As with the Lordmatic 5606 discussed here, the movement is released by rotating the movement retaining spring anticlockwise by about 30 degrees and releasing the crown and stem by depressing the setting lever, visible in the photo above in line with the 4 o’clock marker.

Contrary to the impressions given externally that a pristine interior should await, inverting the movement provides a view of its rear that suggests firstly that moisture has at some point worked its way inside and secondly that it has been some very considerable time since this watch last received the attentions of a watchsmith.

One unusual feature that you may have noticed in the vicinity of the balance wheel is the micro-adjustor.

The regulating screw is accessed via the access port between the lugs and rotation of the screw one way or the other will cause the adjuster to pivot about the lever screw. This then moves the regulating arm of the balance wheel, slowing or speeding up the movement as required. Neat!

Deconstruction starts with the calendar side. This being the date-only 5245, the date dial guard covers the whole of the interior of the movement within the inner circumference of the date wheel. Setting that to one side and we are presented with a layout that contains some surprises but that does not overwhelm in its complexity in the way that, for example, the 5106 did.

Removing the date ring provides an opportunity to get a sense of how this calibre differs from the 56 series and, in particular, how its design deviates from that of every other contemporary Seiko movement that I’ve worked on.

Some noteworthy observations:

1. The one Diafix setting, dial side, uses a shrunken three pronged spring design identical to that of the larger Diashock setting used for the balance arbor. This is the first time I have encountered this design of Diafix.
2. The calendar mechanism uses a cam which provides instant date changeover. This is a similar but slightly less complex design to that used in the 4502 and considerably less complex than the design used in the 5106.
3. The quickset mechanism employs a date corrector wheel rocker similar to that used in the 56 series but in this case, the wheel is made of metal which means it won’t break!
4. The barrel ratchet wheel is located on the calendar side of the movement and secured into position using a reverse threaded screw (signified by the triple slot). This is the first watch I have worked on in which the ratchet is located on the dial side of the movement.
5. In spite of the multitude of advanced features, the layout is clean and simple with very little in the way of unnecessary over-complex design.

The balance side of the movement, stripped of its rotor, reveals the secret of its slim-line dimensions.

In common with the 8305, 5106 and 5626 movements described elsewhere, all of these automatic movements eschew the celebrated magic lever autowind system used in many of the other Seiko automatic movements of the time, and instead employ differential wheel autowinding mechanisms located at the same level as the gear train. The automatic device framework separates from the barrel and train wheel bridge to reveal the first reverser idler and differential wheels.

We’ll review how these parts work later on when we come to reassemble the movement. Lifting the lid further provides a view of the wheel train.

In common with the general approach that seems to have been taken with its design, this movement makes do with fewer components in transferring power to the balance than in the 56 series but the layout is somewhat unconventional. The centre wheel transfers power to the sweep second wheel via the third wheel but contrary to the name given to the former, it is the third wheel that drives the separate sweep second pinion, held in place by its own friction spring. The sweep second wheel transfers power to the escape wheel that in turn drives the balance.

Our final port of call before concluding the disassembly is the seconds stop lever.

Its location appears somewhat alarmingly to display potential for interfering with the escape wheel but in fact it operates on the balance wheel.

We’ve reached our turning point with no obvious causes for concern other than the generally filthy state of the movement as a whole. At this point I would normally be attempting to extract the main spring from its barrel but the technical manual instructs that it is unnecessary to disassemble the complete barrel and arbor. However, it does say that if this is done inadvertently, that the barrel can be lubricated with Seiko oil S-3. The age of this watch persuades me that I need to act inadvertently and so, with some minor struggles, I extract the mainspring.

We start the reassembly process by grappling with those tiny and very fiddly Diafix settings. I tried a number of different approaches to oiling these settings, including pre-oiling the cap jewel but in the end the tried and tested post-lubrication with the cap jewel in place worked best.

With one of the settings, the spring had been flattened and exerted too much pressure on the cap jewel causing it to slip out of position during my attempts to refit the spring. In the end, I had to abandon that spring and source a replacement from a spare parts movement.

The mainspring installs clockwise when viewed from above and this means that, in principle, a left-handed mainspring winder is required. However, I managed to wind the spring backwards using a right-handed winder and installed the spring into its barrel without difficulty.

Reconstruction proper starts with the setting parts, dial side, followed by the centre wheel, escape wheel, hacking lever and its spring, balance side.

The centre wheel and hacking lever are secured into position with the train bridge and the first reverser idler and second reduction wheel lubricated.

The barrel, sweep second pinion, third wheel, sweep second friction spring and sweep second wheel are installed next, in that order.

Next comes the auto-winding mechanism. The second reduction wheel is permanently fixed to the mainplate and needs only lubrication before installing the first reduction wheel and differential.

The purpose of the differential is to translate both clockwise and anticlockwise rotation of the winding weight into a constant direction of the second reduction wheel whose roll is to wind the mainspring in the barrel.

The barrel and train wheel bridge secures all of the gear train into position and requires a degree of jiggling to get everything to seat correctly.

Notice that the barrel arbor is jeweled on this side of the movement. The automatic framework secures the autowinding parts into place at which point we can fit the balance.

The last order of business on this side for the moment is to fit the microadjuster.

Normally at this point, we’d be able to wind in some power to check that the movement runs but we remember that the ratchet wheel is located dial side and so first we have to attend to that.  We start by fitting the click and the sliding crown wheel springs, the latter requiring careful handling.

The ratchet wheel is secured into position with its left-threaded screw and the remaining calendar parts follow.

I’ve highlighted in the photo above the sliding crown wheel, which acts against that spring we installed a step back, and the crown wheel that is permanently fixed to the underside of the minute wheel bridge. The hand winding facility in this movement works as follows: when turning the crown clockwise, the winding pinion on the stem meshes with the crown wheel, turning it in an anti-clockwise direction when viewed from above. The crown wheel meshes with the sliding crown wheel, turning it in a clockwise direction that simultaneously has the effect of pushing it towards the centre of the movement and forcing the sliding wheel to engage with the teeth of the ratchet wheel. When the crown is turned anticlockwise or when the automatic winding is operating, the sliding wheel is pressed against its spring and it no longer engages with the crown wheel.

The 5245 uses a similar design to the 56 series in providing a quickset calendar function in that it makes use of a day-date corrector wheel rocker. I understand that early incarnations of this feature in the 5206 may have displayed similar vulnerabilities to the 56 series if attempting to quickset the calendar during the date changeover period. However, in the 5245/6 movements, the wheel on the rocker is made of metal and no such vulnerability exists. Before fitting the wheel, we lubricate it at the position indicated in the photo below.

The arm of the corrector rocker locates into the profiled hole in the setting lever.

The date jumper comes next and it and the corrector rocker are then secured by the date jumper guard.

With the intermediate wheel, date cam and finger fitted, we are ready to fit the date ring and date dial guard.

It is only at this point in proceedings that we can confidently test the operation of the hand winding, calendar changeover and calendar quickset functions. Thankfully, all of these operate flawlessly. The final step before refitting the dial and hands is to fit the winding weight. You will see in the photo below that it looks a good deal cleaner than it did before service.   I was reluctant to use any sort of abrasion to clean off the oxidation and so gave my salty white vinegar treatment a try and this worked a treat. A good rinse and scrub followed and, although it is not as good as new, it is at the very least perfectly presentable.

We probably ought to check the timekeeping and regulate as needed.

That looks ok. The amplitude drops a little as it settles and with the movement oriented vertically but with some tweaking I am satisfied. We can now move on and fit the dial to check the operation of the calendar functions with it in place.

You can see that there is some wear to the edge of the dial that was not so conspicuous with the original crystal in place but otherwise the dial is in excellent shape.

The case cleaned up very nicely but we still need to attend to the fine adjuster gasket. In keeping with expectations, the original gasket is hardened and no longer fit for purpose.

The aperture hole is of a different size to that in the 5626-7000 and the gaskets I sourced for that watch will not work with this. After some experimentation, I find a suitable small crown gasket that does not creep around the outside of the screw as it is tightened down.

With the hands cleaned of their drying marks and refitted to the movement, we can now locate the movement into the case, fit the crown and stem (together with a fresh crown gasket) and rotate the movement spring back into its locked position.

The crystals for this movement are not so easy to find but I managed to source two from Yahoo Japan sellers. I’d actually forgotten I had done so and selected the first that came to hand which is a third party service part rather than a Seiko original.

The part fitted nicely though, although the inner diameter of the metal ring is slightly larger than the original which exposes a little more of the wear to the edge of the dial.

I think it looks fine though. I can always swap in the Seiko OEM crystal when I have a spare moment.  Here it is with the bezel fitted together with a suitable strap.

I like this vantage point because it disguises just how exotic this case design is until you flip it over.

So what did Daini bring to the table with the 5245 that Suwa had not already covered with the 5625?

Ok, it’s marginally slimmer and therefore perhaps a bit better suited to smaller cases. It possesses every feature of the 5625 and trumps those with instant date changeover. And its timekeeping appears at least as good. Under the hood for me, the 52 series edges it because the more linear design philosophy. By this I mean that the designers have achieved their remit without recourse to over-complicated engineering solutions.

The truth of the design integrity of this movement is evidenced in Seiko’s decision to resurrect it in the 1990’s during the period in which the higher-end mechanical watch movement began its recovery from the quartz domination of the previous 20 years. Developments of this ‘new’ 4S15 calibre went on to find applications in a number of different watch ranges including in Seiko’s high-end Credor sub-brand.

As for this particular watch, I really like the overall design but it is probably a smidge too small to find itself as a regular in my wrist. However, I really, really like the movement. It is top class and I’ve thoroughly enjoyed getting to know it.