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Seiko caliber 6R15 - the heart of many great watches 

Ever since Kintaro Hattori made Japan’s first wristwatch in 1913, the quality of Seiko watches could measure with top Swiss manufacturers. But in 1977, another visionary at Seiko came up with the idea of “the everlasting watch”. Around 600 prototypes and almost 30 years later, Seiko made his famous Spring Drive that made the difference among mechanical watches because it brought a new level of accuracy.

Although Seiko takes pride in some of its old calibers and continues to resurrect them, they never stopped introducing the new ones. And before we analyze the characteristics of some of their top ones (such as the prized caliber 6R15 from the title), let us first break down what exactly is a caliber and why it is important, for those who are less familiar with the term.

In the wonderful study of time called horology, a caliber (also known as movement) is the mechanism of a watch. The origin of the term can be tracked down to mechanical watches, whose calibers are made of many moving parts. The same caliber is often used for many different watches (and as you will see below that caliber 6R15 is one of the most used in Seiko’s watches). Each movement is made of its power source, wheel train, escapement and oscillator.

There are several types of movements. Full plate movements are the oldest and were used in earliest watches, mostly pocket ones. Three-quarter movements were somewhat thinner than former and used in 18th century. Bridge movements, also called three finger or Geneva movements were used at the very beginning of 20th century and when it comes to modern watch calibers, they are classified as manual and automatic movements.

Manual or hand winding calibers need occasional turn of the watch crown to wind the mainspring in order to store the energy for their work for some time. Automatic or self-winding calibers (to whom caliber 6R15 belongs) are the most common today and their mainspring is powered by the motions that one naturally makes while wearing the watch. Therefore, there is no need to manually wind these calibers. The calibers are typically marked with their model number, often together with the design number, such as caliber 6R15-02T0, which marks the back of the case of Seiko Sumo SBDC027 Prospex Diver Automatic 50th Anniversary Limited Edition.

When it comes to Seiko, there are several calibers that marked its history. For example, caliber 6217 incorporated in Seiko Diver’s 150 m in 1965, caliber 6139 made famous by Seiko 5 Sports Speed Timer in 1969 and caliber 35A in Seiko Quartz Astron in 1969 are all great old-timers. But those calibers that made the difference to the modern age followed the introduction of a previously mentioned Spring Drive movement and caliber 6R15 is one of them.

By some, caliber 7S26 is Seiko’s best earner and it was used in several Seiko divers’ watches, but the three most popular Seiko calibers nowadays are surely caliber 6R15, 4R35 and its close relative 4R36. All of the famous Seiko calibers above are parts of so called 7S family because they are based on 7S26 movement.

But, ever since caliber 6R15 is produced in late 2005, it proudly stands as one of the Seiko’s best and most commonly used calibers in modern watches.

Here’s why.

Seiko caliber 6R15 basically adds hand winding and hacking mechanism to 7S26 and is yet less expensive than most of the mainstream Seiko movements. It operates with 23 jewels, beats at 21,600 vph and holds magnificent 50 hours of power reserve with +25/-15 sec/day, which makes it quite a premium workforce. It is just a step below “high-beat” movements, but has greater power reserves and a great accuracy (compared to the “high beat” calibers) due to its Spron 510 mainspring. Seiko caliber 6R15 became famous by Seiko’s SARB (Spirit) series on Japanese market and its fame went global with Seiko’s “SUMO”, “Alpinist” and “62MAS Re-edition” watches series a bit later. 

Even though Seiko introduced newer calibers after it, caliber 6R15 is still one of the most appreciated ones. Nowadays, due to its great accuracy and durability, caliber 6R15 is running some quite serious pieces from SUMO, 62MAS and SARB series. As for the SUMO series, caliber 6R15 runs some magnificent examples such as:

  • Seiko Sumo SBDC027 Prospex Diver Automatic 50th Anniversary Limited Edition
  • Seiko Prospex Thailand Limited SPB029J1 Silver Sumo 200m Diver
  • Seiko Prospex Sumo Pepsi Bezel SBDC057 Diver 200m
  • Seiko Prospex Japan Limited SZSC004 Green Sumo 200m Diver
  • Seiko Prospex Sumo PADI SBDC049 Diver 200m Limited 1000 Pcs.
  • Seiko Sumo SPB055J Zimbe Limited Edition Series 4 Automatic Watch

Basically, it completely rules the Prospex line, from the watches on the affordable price end from $400-ish and up and as of 2018 is used in Seiko Prospex SBD series with price range starting at $600, up to some high-end pieces.

SEIKO watch PROSPEX 1st Diver SBDC053, 62Mas re-issue

That's why upper mid-range watches such as 

  • Seiko Prospex PADI Special Edition SPB071J1 Divers 
  • Seiko Prospex Diver Automatic SPB051J1 
  • Seiko Prospex SPB053J1 Blue Dial 
  • Seiko Prospex Limited Edition Sea Green SPB081J1 

also have the 6R15 core in themselves. The prices of these specimens all go beyond $600 at the Amazon.

When it comes to the SARB series, 

  • Seiko Alpinist SARB017 Automatic Watch
  • Seiko SARB033 Mechanical Automatic Black Dial Watch S
  • Seiko SARB035 Mechanical Automatic Ivory Dial Watch

All these SARB run on the same "engine" and can be found at the Amazon from slightly below $500.

So to resume, caliber 6R15 is one of those so called power horse calibers, which means it works almost like high-beat calibers, but saves power longer. When I say “almost” it means that you probably won’t even notice the difference and the watch is much more affordable. By reading tons of users’ comments on both Amazon and watches related forums, it seems like it is not only Seiko who is in love with this great watch movement.

Seiko Mechanical watches SARB033, SARB035 replacement band 

So if you want your time to be measured as expensive, without it being too heavy on your wallet, Seiko caliber 6R15 is the one to go with, especially when it comes to some of the magnificent timepieces from the 2018 Prospex line.

As calibre 6R15 is by now incorporated in quite impressive number of great timepieces, I’m sure everyone can find just the right fella to do, look and feel great on one’s hand and to serve as a perfect companion in every wild adventure.



All the reasons above are exactly why Seiko doesn’t intend to leave the 7S series of caliber just yet, but rather strives to improve it. Along with some 4r caliber improvements that build some lower end watches, 6r series is in its full bloom, it seems.

For example, besides Seiko’s high-end and limited editions watches presented at Baselworld 2019, some new Prospex models had their premiere. Their heart is made of the new 6r35 caliber, which is basically improved 6r15.

Seiko’s new 6r35 caliber operates on 21,600 bph and has a hand-winding capability, as the 6r15. It is a bit thinner than 6r15. When it comes to jewels, 6r35 has one additional jewel (so it counts 24) and the crystal has been replaced with sapphire. Plus, 6r35 has an additional upgrade to 6r15 because it has astonishing 70-hour power reserve (that’s 20 hours more than 6r15)!

The new Seiko Sumo pieces that saw the light of day at Baselworld 2019 come in two colors: Black (model SPB101J1) and Green (SPB103J1). Their price is somewhat above $900 so the upgrade completely justifies it.

 Another exceptional high-end pieces with 6r35 inside are Seiko Presage referenced SARX061 with a see-through case back (available in the Japanese market only for around $1600) and Seiko Presage referenced SPB095 with Arita porcelain dial that presents exquisite Japanese craftsmanship and is about to be released in September 2019. Its estimated price is around $1700.

In the end, when you take all the facts into consideration and try to compare watches with 6r15 and 6r35 inside, it is obvious that 6r35 wins the race in terms of features and represents the future of 6r15. That may justify the difference in the cost of timepieces it runs, but it doesn’t mean we should forget all these great watches run by 6r15.


orignal image from neel.co.jp

6r15 still serves the purpose and counts as a reliable power horse. So it’s really hard to give the verdict here. At the very first thought, 6r15 seems like an everyday outfit compared to 6r35 – meant for special occasions, but in reality, a lot of other factors need to be considered. New Prospex and Sumo are great watches made for adventures, and then there’s the new Presage – the stylish fella full of charm in any of its versions. But then again, 6r15 covers the same range of styles and has some really handsome contestants here.

So maybe it’s best to just go with what you like. Let’s be honest, you won’t go wrong by choosing any of the watches run by either of these two calibers. So take a good look at each of these timepieces and pick one(s) that appeal most to you. Because whatever watch with these calibers inside you pick, one thing surely stands: its heart is good – because it has the “genes” of 7S family.

And the heart is important.


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Photo credit: @gioielleriaangelini / @seikowatchofficial / @neel.co.jp

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