Seiko’s Marine Master 300m diver is the direct descendant of the 300m 6159-7001, Seiko’s first professional dive watch released in 1968. The design has changed little over the years, it still utilises a monocoque case and a screw down crystal with a crown at the 4 o’clock position. The original was powered by the high beat 36,000bph calibre 6159a movement whilst it’s grandson uses Seiko’s fantastic calibre 8L35B, which is a 28,800bph, 26 jewel, hacking, handwindable, automatic movement with a date display. This example suffered from a clout against a door frame and the timing went a bit haywire so it was decided whilst the movement was out of the case it may as well have a thorough service.

As previously mentioned these watches use a one piece case, they still have the tsunami logo on the back though.

With the bezel and the crystal assembly removed the movement the stem latch can be pressed to release the crown/stem assembly.

Once the crown is removed the movement can be carefully lifted out.

With the movement turned over the rotor can be removed, the rotor screw needs a special tool to undo it.

With the rotor removed the compact movement is revealed.

I turned the movement back over and removed the dial and handset. Seiko use 4 crosshead screws to secure the calendar top plate, I’m not sure why slot heads weren’t deemed good enough though!

I then removed the top plate and calendar ring, this left the calendar wheels and keyless work on display. The keyless work is quite a departure from the more usual Seiko fare with its rocker beam with four wheels attached.

Once I’m in a bit further in another difference is the canon pinion, it’s similar to the type used in Seikos 7123 movement where the wheel’s crimped to the bottom of the minute tube. It also has to be removed from the motion work side instead of this side.

Soon the calendar side was fully stripped….

….which meant I could turn the movement over and start on the other side.

The balance was removed out of harms way first.

The autowind work is different to the usual Seiko form, it still utilises the pawl and transmission wheel but the transmission wheel rotates on the ratchet wheel driving pinion, a very compact solution!

I then removed the bridges which meant the train was ready to be removed.

This is the view of the underside of the train bridge, you can see the manual winding transmission wheel and the brass coloured pawl.

This left the final bridge to be removed, once this was off the second wheel and canon pinion could be taken out.

The parts were put through the cleaning machine and inspected.

The reassembly began with the mainspring as usual for me.

The escape wheel and pallet fork are worth a closer photograph, it’s still a Swiss lever club foot escape wheel but the lattice work looks lovely!

The first couple of wheels and bridge went back together.

Then the train was put back….

….and the bridge refitted.

The pallet fork, balance and autowind mechanism was next to be reassembled, this meant the watch could now have it’s timing roughly set on the timing machine.

The calendar side was tackled next….

… the date ring back on….

….then the top plate.

The movement was turned over and the casing ring and the rotor were refitted….

….then it was turned over again and the dial and handset were replaced.

This was then dropped back into the case and the crystal assembly was refitted….

….then the bezel was replaced and the job was done. The MM300’s a cracking tool watch and can be adjusted and regulated to be amazingly accurate, it’s on it’s third day on test now and hasn’t gained or lost a second in that time.