What is Seiko Tuna?

 

In the 1960s, both Swiss and Japanese watchmakers 'worked out' that when watches were used professionally for helium saturation diving at great depths, they blew their glasses off during decompression. The problem was - Helium. Under high pressure, the inert Helium gas could enter the watch through regular gasket constructions, building up pressure inside the watch, equal to the pressure outside of the watch at that depth. Then, while decompressing the diver, the helium gas couldn't get out quickly enough, and thus the build-up pressure inside the watch caused the glass to blow off. In order to allow for quick helium release, the Swiss decided to construct a case with a helium valve.  The Japanese opted for another solution: to construct and build a divers watch so 'tight' that the helium couldn't enter the watch in the first place. Seiko's solution was found in a special type and construction of the gasket, combined with titanium monocoque casing. The first Tuna was built in 1975, fitted with an automatic movement, waterproof to 600m.  The rest is the history. Three years later, another release: the new quartz Tuna. It was the world's first professional Diver's 600M featuring a world-class corrosion-proof, shock resistant, airtight case.  In 1985 Seiko released a 1000m titanium Tuna with ceramic bezel protection fitted with a high grade quartz mechanism. This was not an ordinary battery operated watch, but one fitted with a purpose built mechanism: shock resistant, high torque, and super accurate. The diver's watch standard - robust, reliable, repairable - was set by Seiko. During the past couple of decades the market for divers watches exploded. New models offered everything from a dive computer to email and web browsing to banking. Yet only a handful of watches were still built to 1980s standard: a true 1000m water resistance.  In 2020, the new-old Tuna was welcomed by tens of thousands of serious watch aficionados looking for 'an old fashioned, properly built, no-nonsense' divers watch. A classic watch featuring high-tech materials like titanium, coated steel, and ceramic. A repairable watch which could be passed on to the next generation.

Inside the Tuna can

The original plan was to publish here a couple of images to show the 7C46 movement as well as the single-piece titanium case. However, since no suitable images could be found, I has no choice but to dig into it myself. The disassembly starts with removing the 4 side screws with a 1.5mm hex key. Once the screws are removed, the ceramic 'shroud' comes off, as well as the rotating bezel.  Underneath the bezel are two click springs which hold the bezel in position.

At this point in time, the watch is still fully waterproof, so removing the bezel protector for cleaning or for the replacement of the click/bezel itself does not affect water resistance. (Hint: you can clean the bezel yourself!)

Crystal removal requires a special tool. The crystal is held down by a screw-down bezel. Between the bezel and crystal there is a poly washer.

The crystal's rubber seal is of a special shape, designed to 'hug' the crystal when pressurised. The crystal itself is a sapphire, flat, faceted, 4mm thick.

The next step is to remove the minute chapter ring. Underneath the chapter ring is a rubber O-ring which provides shock-absorbance for the dial and movement.

Stem removed, movement extracted form the case.

7C46B is a high torque 7 jewel movement specifically made as a Seiko 'diver's calibre'. Operating range: between -10 ºC and +60 ºC. Accuracy: Less than 15 seconds per months when worn in temperature range between 5 ºC and 35 ºC. Or spot on when worn at the wrist! Again, these are impressive figures: 70 degrees operating range and 30 degrees guaranteed accuracy band. 7C46 is a fully repairable mechanism.

The monocoque, a single piece case.

The heart of the Tuna is the single piece case, machined entirely from a solid titanium block. No case back here! The quality of the case is simply superb, from finishes to tight tolerances. Indestructible, corrosion free.  If you are into machining and manufacturing, you will be impressed: this is not a stamped-out mass produced case, but a one-at-a-time CNC high tech machined component of 'medical grade' quality.

Reassembled, and water pressure tested. Unfortunately my water pressure tester goes only up to 10 bar. Tuna easily withstands the ten times greater pressure of 100 bar!

Attention to detail: divers safety is important! On the back of the case, a scale of 25 to 35 can be found. And the famous 'dot': the punch mark carefully placed, indented by hand. Your Tuna needs a new battery in the third quarter of 2025! Such a personal touch... Who else would mark a perfect, brand new titanium monocoque case - but Seiko. Simply saying: this is a tool watch, not a toy.

In conclusion Tuna is much more than a serious watch. It is a high tech professional divers watch and a significant historical reference to Seiko's engineering and watchmaking. A watch entirely made in Japan, by the only company in the world that makes every single watch component in house, under one roof. This piece needs no recommendation: if you are in or around the water, looking for a watch you'll never have to take off the wrist, a recognizable piece which will generate comments by those who know their stuff, then you can't pass the Tuna. Boutique price: $3,750.  As said before, my promise to you, my subscribers, is to offer you unbeatable deals on the most sought after and most iconic Seiko out there. I do believe that Tuna is worth every cent of the $3,750 retail price.  Model Reference: S23631J Case size: 49.4mm over the bezel protector, 42mm measured over the bezel.