I’ve written about these JDM 6306 dive watches before but it’s worth a revisit just to showcase what beautiful condition this example from 1978 is in. It came in recently from a well known Seiko enthusiast for a movement service and a pressure test. It’s in outstanding cosmetic condition for a 37 year old wristwatch, the only negative points were the reason it came in, it needed a service! The amplitude was low and positional variation and beat error were a little high.

The caseback reveals a production date of August 1978 and again it’s unblemished by careless opening.

With the back removed the automatic, hacking, 21 jewel, 21,600bph movement is revealed.

Once the movement is uncased and the hands removed you can see the excellent condition of the dial. In real life the plots are starting to turn a lovely creamy colour but my harsh lighting tends to wash this out.

With the dial and day wheel removed the calendar/keyless work is revealed.

The simple but effective keyless work is about to be removed here.

And the calendar side is fully stripped here.

Turn the watch over and the auto work has been removed revealing the two extra jewels and caps that the 6306 has over the 6309. These are called diafix jewels by Seiko and afford shock protection to the third and escape wheel.

With the trains bridge removed you get a view of the hacking lever running under the third and fourth wheels, again the 6309 doesn’t have this feature.

The movement was soon fully stripped ready for cleaning and inspection.

No problems were found during inspection so the rebuild began with the mainspring as is usual with me.

The train coming together here.

The bridge back on and those fiddly diafix caps and springs are safely back in! It’s at this stage that I put some power in the spring and roughly adjust the timing and balance parameters.

The autowind bridge refitted.

The calendar side coming together.

The date and day wheels back on, JDM models always have the kanji script option on the day wheel.

The dial and handset refitted.

The movement was then recased ready for closing up. The seals looked to have been replaced recently, you can tell quite easily with the crown as a fresh seal is tight to fit over the tube.

Now she’s back together it’s time for a swift pressure check to verify it’s OK to get wet.

With that successfully passed she’s finished and ready for a few more years service!