Mechanical Heart: A Close Look into Seiko's Watch Movements

A watch movement is the mechanism that keeps time, and actuates the hands of a watch. In a digital watch, it controls the digital display. The accuracy of a watch movement describes how closely it is able to match an atomic clock, which is generally recognized as the most accurate form of time keeping.

Without a movement, a watch is a fancy piece of machined metal (or other material), with no practical purpose. Watch movements come in many different kinds, and there are a range of manufacturers that produce these mechanisms.

Credits: Satya Rasyid Triabadi on YT

Watch movements have evolved over the centuries, and today, there are a multitude of movements to choose from on the market. One maker that has created consistently good movements for more than 100 years is Seiko, which, as a manufacturer, started off as Seikosha in 1891.

Today, there are two major kinds of movements, automatic and quartz.

An automatic watch uses a fully mechanical system to keep time, while a quartz movement uses a small quartz crystal to achieve timekeeping. Some watch movements rely on handwinding, and like other exotics, don't really get used all that often.

By the end of this blog, you will be able to easily tell which is automatic and which is quartz. (Source: Crown and Caliber)

In a mechanical watch, there is a spring that is wound either by hand, or by a small weight that spins as a person moves during their day. As the spring unwinds, that energy is used to drive a small machine that keeps time.

For quartz watch movements, a battery is used to apply a small current to a quartz crystal, which vibrates. This vibration is very uniform, and can be used to create the basis of timekeeping.

There are also other kinds of hybrid movements, like Seiko's Spring Drive, or Citizen's Eco-Drive. We are going to focus on basic movements in this article, as they comprise the vast majority of watch movements that are produced commercially every year.

Watch Movements – The Basics

The vast majority of watches that people collect are automatic. The reasons for this are nuanced, but after the quartz revolution of the 1970s, most watch collectors shifted to automatic, mechanical watches.

While quartz watches were initially very expensive, much like pocket calculators, quartz watch movements fell rapidly in price. Today, you can buy a digital watch that has a rock-solid quartz movement for less than $100 USD.

SII's VK63A Quartz Movement retails for as low as USD24.99. (Image Source: Ebay)

No one really knows why mechanical movements have remained so popular, as they are both more expensive than most quartz movements, and also less accurate. A high-end Japanese or Swiss automatic movement may be accurate to +/- 5 seconds a day, while a quartz movement like the Seiko 9F will likely be accurate to +/- 10 seconds per year.

That said, many people like the complexity of an automatic movement, and when regulated, a modern Japanese or Swiss movement is accurate enough for daily life.

Japanese vs. Swiss Movements – What's the Difference?

The Japanese and the Swiss have been competing for watch movement supremacy since the 70s. Depending on who you ask, there may or may not be a big difference between Japanese made watch movements, and Swiss movements. To be sure, Japan produces many more lower-end movements, while the Swiss concentrate on the luxury watch market.

You can see by the overly complicated design that the Swiss does not only create movements; they create art. (Source: WatchUSeek)

This isn't to say that Seiko doesn't craft outstanding timekeeping mechanisms, they certainly do, but the vast majority of the offerings from Japanese manufactures cost well under $1,000 USD. That can't be said about Swiss watches, and along with a higher cost up-front, service for Swiss models also tends to be much higher.

Some people think of the Swiss as more innovative than the Japanese, but this simply isn't the case. Seiko was the first company to manufacture a quartz chronograph watch with an analog display, and Japanese innovation nearly put the Swiss out of the watchmaking business in the 1970s.

Of course, it is pretty easy to just look at an entry level Seiko automatic, like the 7S26, and see how different its appearance is from a Swiss movement like the ETA 2824. A Seiko 7S looks like a commercial product – the level of finishing is spartan, and it is built by machines.

Seiko went for function over form for the 7S series of movements and produced great results. (Source: russell_w_b on Flickr)

On the other hand, most Swiss movements are given some level of hand finishing, and as the prices go up, the level of finishing and technology also rises. Whether or not you see value in this is a personal preference, as even a regulated entry-level 4R35 will post performance results that rival Swiss movements that cost many times more.

Seiko: A History of Making Quality Movements

Seiko has been making watch movements for over a century, and it has created some of the most innovative movements in the history of modern watchmaking. In fact, one could argue that Seiko was responsible for starting the quartz revolution.

In 1969 Seiko introduced the world's first quartz watch, which is called the Seiko Quartz Astron. The Astron was far more accurate than the mechanical watches of its day, and kept accurate time to within 5 seconds per month. It was an amazing watch, and helped Seiko cement its place at the top of the modern watchmaking pantheon.

The watch that started the whole quartz revolution of the 70's, the 1969 Seiko Quartz Astron. (Source: Hodinkee)

While Seiko did receive praise for its Quartz Astron, many companies entered the Quartz movement market which helped popularize the technology. However, many people still preferred mechanical watch movements, which do have some advantages over quartz.

Seiko never abandoned its mechanical watch movements, and in fact, created one of the most popular series of mechanical movements decades later. Unlike some of the Swiss movements, Seiko's 7S line has always been an affordable workhorse that powered the iconic SKX divers.

The 7S family of movements includes the famed 7S26, but also the 6R15, 4R35, and 4R36. All of these watches share some basic design ideas, but they are all a little bit different. Without a doubt, the 7S26 has been widely bought as the heart of the SKX despite lacking some modern features.

The 7S Base is Very Popular

The 7S series includes the 7S25, 7S26, 7S35, 7S36, and 7S55, all of which have been used in various Seiko Models. All of the movements in the series lack hacking and handwinding, and also use a traditional mainspring.

The 7S26 was introduced in 1996, and will hold a 40-hour power reserve. It is a 21-jewel movement, and its daily timekeeping specs are -20 to +49 seconds, which isn't nearly as good as some of the other movements in the 7S Series.

Quick Fact: Jewels are added to watch movements to reduce friction between moving parts, increase accuracy, and increase the life of the bearings. They are usually made of synthetic sapphire or synthetic ruby.

The anatomy of the 7S26C Movement (Source: Seiko Service USA)

It does beat at 21,600 vph (vibrations per hour), which puts it in the same class as many other tool watch movements.

One of the biggest ways the 7S26 found its way into the market was inside of the SKX007, and the entire SKX line. While the SKX was originally introduced in the 1960s, the reintroduced 1996 version would carry the 7S26 into history as one of the most popular automatic movements.

A Big Upgrade: The 6R15

The 6R15 shares a lot with the 7S series, and many think that is the modern upgrade to the classic Seiko moments that came before it. While it is a date only movement, as it lacks a day indicator, it offers some major upgrades over the 7S series.

To begin, the 6R both hacks and handwinds. In addition to these modern features, the 6R also adds a Spron 510 mainspring, which helps to push the power reserve from the 40 hours of the 7S26, to 50 hours for the 6R15.

The Spron 510 alloy is actually made by Seiko, and is an alloy of cobalt, nickel, molybdenum, among other elements that help give it extra performance characteristics.

The famed 6R15 which powers the likes of the Seiko Alpinist SARB017 and the Seiko Sumo SBDC027. (Source: Caliber Corner)

Like the 7S series, the 6R vibrates at 21,600 vph, although it has 23 jewels, opposed to the 21 of the 7S26. It has a rated accuracy of +25/-15 sec per day, although many owners state the accuracy is much higher.

The 6R series of movements powers Seiko models like the Sumo, Alpinist, SPBXXX diver, as well as the extremely popular SARB line. While the 6R series is a very popular line of movements, it is not used extensively in the Seiko modding community.

Seiko recently introduced the 6R35, which pushes the power reserve of the movement up to 70 hours, and also adds a jewel, taking the total jewel count to 24. It isn't an inexpensive movement, and is used on the higher end of Seiko's midrange watches.

The NH35 and NH36: The Movement of Choice for Seiko Mods

The 4R35 and 4R36 are in many ways a hybrid between the original 7S series, and the higher end 6R variants.

Both of these 4R movements are built from the 7S base, and add both hacking and handwinding. Seiko manufactured unbranded versions of these two movements, which are the NH35A and NH36A. These are used mostly by microbrand watches like the Undone Aero or the Dan Henry 1970 Automatic Diver.

The Undone Aero is a fun semi-custom watch powered by an NH35A Movement. (Source: Wrist Watch Review)

While the NH35 only has a day complication, the NH36 has both the day and the date, which makes it a perfect upgrade for the original 7S movement. Because it is based on the 7S, all the original SKX and aftermarket dials can be used with the NH36 without any modification.

The operating specs of the NH series are also very similar to the 7S, but with some added performance. The NHs beat at 21,600 vph, and both have a stated accuracy of +45 / -35 seconds per day – with that said, in practice, the NHs can post much higher accuracy results, and can also be regulated for better performance.

An illustration of the NH35A. (Source: Seiko Instruments Inc.)

The vast majority of Seiko mod parts are made to fit SXK cases and movements, which makes the NH35/NH36 the ideal choice for a movement that offers modern features, and backward compatibility. Many Seiko 5 mods also use the NH series for this reason.

If you have an old SKX, and you want to update both the looks and the movement, the NH36 is a perfect skx movement mod to go with some new parts for the watch. It can use the original hands, and will give you a big upgrade in functionality over the 7S that came in the watch.

Why Should You Use the NH Series

The NH35 and NH36 offer modders loads of options, and are totally compatible with Seiko SKX cases, as well as cases like our NMK901, which is often paired with the NH36. These movements are also totally compatible with our wide range of aftermarket dials, and can be used with date only versions, if you prefer using the NH35.

If you are looking for a smaller case to use as the base for a Seiko mod, our new NMK912 is a perfect fit. It offers plug-and-play functionality with the NH35/NH36 series, and can be used with numerous parts that we offer for Seiko diver builds.

Our NMK912 case with the SPB143 dial and powered by an NH35. (Source: John Doe of SE)

While the NH35 or NH36 movements are native 3'o clock movements, they can be used with a 4' o clock dial, when a day wheel is transplanted from a 4' o clock Seiko movement, like the 7S26. We also offer a Kanji Black font version of the NH36A, which looks great with blacked out Seiko mods.

Can you think of mods to use our NH36A Black Kanji Version with yet? 

Our entire collection of crowns is also compatible with both NH movements. You just need to trim them down to get the perfect length for your mod, and you’re good to go!

For a prime quality Seiko movement, and loads of other top-tier aftermarket Seiko compatible parts, NamokiMODS has you covered. In addition to aftermarket SKX cases, we also have many case designs that allow you to use SXK compatible parts with cases that give you more style options for your next Seiko mod.

Seiko diver watchmaking has come a long way since the 1960s when it began, and we can help you get the most for your money – NamokiMODS has many parts that can help you improve on the OEM equipment that Seiko supplied, like a sapphire skx crystal, or new NH36 movement. Have a look at our website for more information!