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By rileynp


The following post is a culmination of spending too much time here, silently soaking in all the ideas floating around, and being spurred on by them. I found the SCWF during my days at watchmaking school, over 5 years ago. Since then I have quietly followed, more closely than was probably healthy (ask my wife). But there was (and still is) a lot to learn here, that has shaped much of what I know (and love) about Japanese timepieces, Seiko's in particular.

I would like to share something in return, a project that has its genesis in a relationship initiated by the forum. Read here for another perspective on that story (he remembered more about it than I did):

The evolution of the 6309GMT started with Josh's crazy request to have a GMT movement inside a diver's case. Because of the close relationship that the 61xx and 63xx families share, it was decided a donor 6117 would go inside a 6309 case. As with most any project I undertake, it was not as simple once actually started (but hey, it looked good on paper). Some significant stem alteration and fabrication was necessary to make it work (the profile of the female 6309 diver's stem is different than a 6117 stem, and the male 6309 diver's stem is too short). Being in the middle of marrying a 6117 movement with a 6309 case, I was engrossed in the similarities and differences between the 6117 and 6309. A playful thought entered my head at some point- "What if a 6309 movement could be made to have an additional GMT hand (one that rotates every 24 hours)?" It is a testament to the logical and sound engineering by Seiko's movement designers that enough similarities remain between the 6117 and 6309 dial side (calendar parts) so that one can take a few 6117 parts, modify a few 6309 calendar parts to make room, and there you have it- a 6309GMT, easy-peasy. OK, maybe not exactly, but that's the theory. I did not know for sure from the outset that this project would work; I started with the assumption that it wouldn't, and then examined each aspect in such a way to make it do what it needed to do to arrive at the final outcome. The skeptic's approach.

Following is a break-down of what needs to be done to add one skinny little hand to a 6309 diver's watch. Or a 754x diver's watch. Did I mention the logical and sound engineering of Seiko movement designers? To have a quartz movement based on an automatic is sheer genius, and the implementation of it was carried out very well. To me, the mark of a successful watch manufacturer is not being able to make a small number of really outstanding pieces, but rather to consistently make a large number of outstanding pieces. Not everyone shares this same value (that's fine), but for me this is part of the draw of Seiko. Here is a comparison of a 63xx main plate and a 754x main plate, notice all the similarities. Most likely the rough blanks started out the same, and had many of the same operations performed on them during manufacture, before going their separate ways to becoming 63xx and 75xx main plates. All of the parts attached to the dial side of the main plate are interchangeable between the two:

But I digress. Getting back on topic, here are the external things that needs to be changed/added, First and most importantly,the dial:

While the external diameter and chapter ring tab at 10 minutes are the same as for a 6309 diver's dial, the similarities end there. The center hole needs to be larger, as the pipe for the 24H hour wheel is too large to fit through a standard 6309 dial. Also, the milled relief cut into the underside of the dial is approximately twice as big in diameter, to allow proper clearances and end-shakes for the 24H Hour Wheel (what the 24H hand will be attached to). Lastly, the day disc cannot be used (as the 24H hour wheel is installed where it normally would sit around the standard hour wheel), so a date-only window must be present in the dial. One could go with no date, but why leave out a function when you can just as easily keep it? These three criteria meant that the dial would have to be custom-fabricated- many thanks to Noah Fuller for taking on the project and delivering a great product. Oh, another design feature is that the surface of the dial must be relatively flat, with no raised emblems or markers, to allow room for the 24H hand.

Second, in addition to the regular hour, minute, and second hand, a 24H hand must be acquired:

The hole size is critical for proper installation, but so is the thickness of the pipe as well as the diameter of the flange around the hole (for aesthetics and also for removal's sake when servicing). This brings up an important point- this modification can be serviced just as easily as a standard 6309. My approach was to imagine how Seiko would have made a 63xx movement with a GMT function, and follow that idea. One can look at it like this: the 6105 caliber is to the 6117, as the 6309 is to the 6309GMT. Many thanks to Harold (Yobokies) for taking on the hand project and delivering a great product.

Third, the rotating bezel must be altered to make the GMT function truly useful:

While there were other 24 hour bezel inserts available, this style (shown in the first photograph above) better suited the dial and hands, so it was deemed an integral part of the project (Thanks to Noah Fuller again for supplying it to our specifications). Now, some of you may be wondering about the functionality of a GMT bezel that has only 60 clicks- how do you offset the bezel for the odd hours (0100, 0300 ,0500, etc.) since the marks for these hours fall between minute marks (0100 equal 2.5 minutes, 0300 equals 7.5 minutes, 0500 equals 12.5 minutes, etc)? Well, the answer is that you luck out in the design of the 6309 rotating bezel. Each stop for the bezel click ball is actually a valley of sorts milled into the underside of the bezel, so that the ball springs up into the valley and creates a point of increased friction- felt as a "click" as the bezel is rotated. There is enough room between the 60 valleys to file an additional valley, so adding 12 valleys at just the right spacing makes for a bezel that is truly useful when employing the GMT function to keep track of a different time zone. Other rotating bezel designs I have encountered would not allow such a relatively easy method of modification. Sorry Sir Les, I can't for the life of me figure out where to file to allow one to track Adelaide time:)

Let's get to the even better stuff (to me)- the internal movement changes/additions

First is the dial holding ring (not shown in the photo above)- it's height must be modified to allow enough clearance between the dial and the 24H hour hand, but also to provide proper end-shake (up-and-down movement) for the 24H hour wheel. As with the 6117, the 6309GMT's 24H hour wheel is kept in place by proper spacing between the dial and the wheel, with a dial washer taking up the additional play. The original amount of height of the standard 6309 dial holding ring is designed to leave clearance so that the day disc does not rub the dial, but since the day disc is no longer installed, it is no longer necessary or desirable (as one wants to allow as much clearance as possible for the 24 hour hand above the dial).

For reference, here are the stock 6309 calendar parts before modification:

Next, the date dial guard and day jumper must both be altered to allow room for the 24H hour wheel, which sits on the same vertical plane as they do. I used a combination of filing, grinding, and milling to make the changes. The corrector wheel for the day disc must also be removed from the minute wheel bridge, as it is not used and gets in the way of the 24H hour wheel:

Lastly, the day finger must be altered to allow clearance for the 24H hour wheel, but still hold the date driving wheel in place.

Now that room has been made for the 6117 24H hour wheel, one must find a way to connect it to the rest of the watch so that it is caused to rotate at the correct speed. Hmm, why not use the 6117 intermediate date wheel, which has an additional set of teeth on top to drive the 24H Hour Wheel? Once the day jumper has been shaped to allow clearance for the 6117 intermediate date wheel, it fits right into place and does the job just as if it were in a 6117 movement. The tooth count and effective profiles are the same for all mating wheels (hour wheel, 24H hour wheel, intermediate date wheel, and date driving wheel). Here are the modified 6309 parts shown with the 2 additional 6117 GMT parts necessary (By the way, you will perhaps notice that the plastic components shown in some of the photos switch between green and gray- they are simply duplicate parts from different donor movements. These photos were taken at various times and of various movements. Seiko changed the color [and composition I would imagine] of their plastic components in the 6309- gray was the earlier of the two):

And there you have it, my secret method. I would not have been inspired/prodded to work this out had I not benefited from so much information that had been so readily shared here, and I would like to return the favor in the same spirit. Honestly, I was tempted to keep the details of the process a secret at first (watchmakers can be like that with their own bright ideas- probably because when they share them they find out some one else had the same idea a long time before them), but that's no fun for anyone, really. I hope you've made it this far, and if there is anything I can clarify, I would be happy to do so.

Thank you for letting me share. --Noah Riley
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