Seiko Mod Movements Explained: NH, VK, Miyota, and What to Use in 2026

The movement is the beating heart of every watch, and for those getting into Seiko modding and wanting to build a full watch from scratch, deciding on the right movement or caliber should be done early in the planning stage because it dictates a lot of the other parts you’ll need to use. Whether you're creating your first custom timepiece or you're an experienced modder looking to expand your repertoire, understanding the available movements is something you have to do if you love the craft.

Back then, early Seiko modders used to reuse the 7S26 that came with their SKX007’s, but as it got discontinued and with fewer OEM 7S26 movements available in the market, they moved over to Seiko’s unbranded NH calibers, particularly the NH35 and NH36. With the same reliability and affordability that the 7S was known for while also adding hacking and handwinding capabilities, the NH movement quickly became the new go-to movement option for modding. Since then, there have been more movements that got supported by mod part suppliers, like the GMT NH34 or the Skeleton NH7X movements. It would evolve even further to completely new platforms like VK mecaquartz and even non-Seiko movements like the Miyota 9 series.

In this comprehensive guide, we'll explore the most popular movements used in Seiko modding today, diving into their specifications, real-world performance, and ideal use cases. By the end, you'll have the knowledge to confidently select the perfect movement for your next build.

 

The NH Series: Foundation of Seiko Modding

It wouldn’t be a stretch to say that the NH movement changed the modding scene when it was released over a decade ago. These movements were based on Seiko’s older 7S designs but added features people actually wanted, so people were sometimes upgrading their SKX watches even if their existing 7S calibres still had a lot more years in them.

The NH series sits in a sweet spot. It is reliable enough for daily wear, simple enough for beginners, and flexible enough for experienced builders. That is why it became the foundation of Seiko modding.

All NH movements share the same basic specs:

• 21,600 beats per hour

• Around 41 hours of power reserve

• 24 jewels

• Automatic and manual winding

• Hacking seconds
  

These specs might not sound flashy, but they are proven. NH movements are forgiving, easy to regulate, and tough enough to survive years of use. 

 

NH35: The Workhorse

The NH35 is the most widely used movement in Seiko modding, and it holds that spot for a simple reason. It works. This automatic three-hand movement with a date function gives builders everything they need without adding cost or complexity. Rated at roughly -20 to +40 seconds per day, its real-world performance often matches or beats expectations, especially once it is worn regularly.

What makes the NH35 so popular is how flexible it is. The layout is simple, reliable, and forgiving, which makes it ideal for first-time builders. At the same time, it is fully compatible with an enormous range of aftermarket cases, dials, hands, and accessories. Whether the goal is a dive watch, a field watch, a dress piece, or a clean everyday build, the NH35 fits easily into almost any design. It is the safe choice, but not a boring one.

The date display sits neatly at 3 o’clock and comes with either a white or black date wheel, making it easier to match the movement to your dial. It also includes hacking seconds, which stops the seconds hand when the crown is pulled. This allows for precise time setting, a small detail that makes a big difference in daily use.

Another key advantage is that the NH35 is unbranded. There are no logos or markings forcing you into a specific design language. This gives modders full creative freedom and is one of the reasons microbrands around the world rely on it. The NH35 has earned its reputation through years of proven reliability and wide availability of parts.

 

NH36: Adding Day Functionality

The NH36 builds on that same foundation by adding a day display alongside the date. Everything else stays largely the same, including size, thickness, power reserve, and accuracy. The added day function brings extra usefulness and works especially well for watches meant to feel practical or professional. This movement is commonly used in tool watches, vintage-inspired builds, and dress watches where day and date information feels natural.

One unique feature of the NH36 is the option for Kanji day wheels alongside English. This small detail is popular with collectors who want to lean into Seiko’s Japanese roots and add personality to their build.

Choosing between the NH35 and NH36 mostly comes down to the dial. If the dial only has a date window, the NH35 is the correct choice. If the dial has both day and date windows, the NH36 is required. Mixing the wrong movement with the wrong dial leads to empty or misaligned windows.

One final detail that matters is crown position. Both movements come in 3 o’clock and 4 o’clock crown versions. Using the wrong version will cause the day or date to sit off-center. Match the movement to the case, and both the NH35 and NH36 will reward you with years of dependable use.

 

NH38: Minimalist Open-Heart Design

Source: u/dazedgonebye9988 on Reddit

 

The NH38 takes a different approach from most NH movements by removing the date function entirely. This creates a cleaner dial layout and avoids unnecessary complications. It is not just an NH35 with the date removed. The NH38 was designed specifically for open-heart watches, where part of the movement is meant to be seen from the front.

At the 9 o’clock position, the NH38 exposes part of the balance wheel and escapement. These are the components that regulate timekeeping, and watching them move adds a sense of life to the watch. The opening is subtle, not flashy, which makes it a great fit for builders who want visual interest without going full skeleton.

Beyond looks, the NH38 solves a common issue known as the ghost date. When a movement still has a date mechanism but no date window, the crown can feel like it has an extra click that does nothing. This can feel cheap or confusing. Because the NH38 completely removes the date system, crown operation feels clean and direct.

This movement works especially well for minimalist builds, dress watches, and symmetrical dial designs. With no date window to work around, the dial stays balanced and uncluttered. It is a strong choice for modders who value simplicity and want a watch that feels intentional rather than stripped down.

 

NH34: Bringing GMT to the Masses

The NH34 is one of the most important additions to the Seiko modding world in recent years. Before it existed, building a true GMT-style watch meant expensive movements or awkward workarounds. The NH34 changed that by offering dual time zone functionality at a price that makes sense for modders.

At its core, the NH34 adds a 24-hour hand that allows the wearer to track a second time zone. This is useful for travelers, remote workers, or anyone who wants GMT functionality without spending luxury-level money.

The NH34 shares a similar thickness to the NH35, but there are important differences. The hand stack is taller and the center post is wider to support the extra GMT hand. Because of this, many standard NH35 dials will not fit without modification. Some builders carefully widen the center hole using a round file, while others choose dials made specifically for NH34 builds.

Crystal clearance also matters. The taller hand stack means a flat crystal may cause the seconds hand to rub. Most NH34 builds use a double domed crystal or a case with extra height to avoid this issue.

It is also important to understand how the GMT function works. The NH34 is a caller GMT, sometimes called an office GMT. The GMT hand adjusts independently, while the main hour hand moves together with the minutes. This is different from a true GMT, but for most users it does the job well and keeps costs down.

For GMT homages and travel watches, the NH34 has quickly become the default choice. It brings useful functionality without sacrificing reliability.

 

NH70/NH71/NH72: Full Skeleton Spectacle

The NH70 and NH71 take the open-heart concept of the NH38 and steps it up further, offering fully skeletonized movements that transform the watch dial into a window to mechanical artistry. Built on the NH35 framework, these movements retain all the standard features while exposing the marvels of engineering inside.

The primary difference between the NH70 and NH71 is purely aesthetic. The NH70 features nickel plating for a silver finish, while the NH71 sports gold plating for a warmer, more luxurious appearance. There is also the NH72 which comes in a subtly exquisite gunmetal color. This choice allows modders to match the movement finish to their overall design aesthetic, whether contemporary and cool or classic and warm.

These skeleton movements are designed for Seiko modding projects where the movement itself becomes a design element. Paired with open dials or dials featuring generous cutouts like our Lattice or Snowflake dials, the NH7X creates a watch that's as much a conversation piece as it is a timekeeper. The exposed mechanics appeal to those who appreciate the craft of watchmaking and want to showcase it on their wrist.

 

VK Mecha-Quartz: Bringing in the Chronograph

Not all Seiko mods use automatic movements. The VK series of mecha-quartz movements offers an interesting alternative, especially for chronographs. The term 'mecha-quartz' refers to the movement's hybrid nature. While the timekeeping function is controlled by a quartz crystal for exceptional accuracy (within 15 seconds per month), the chronograph mechanism operates mechanically. This means you get the satisfying sweep of a mechanical chronograph second hand without the bulk, complexity, or cost of a fully mechanical chronograph movement. They are thinner than most mechanical chronographs and require less maintenance that makes them ideal for sporty builds.

 

VK64: Bi-Compax

The VK64 has become the most recognizable member of the VK family, powering numerous microbrand chronographs and custom builds. Its bi-compax layout features subdials at 3 and 9 o'clock: a 60-minute chronograph counter and a 24-hour indicator, respectively. The date window sits cleanly at 6 o'clock.

What makes the VK64 unique among its siblings is the absence of a running seconds subdial. This creates a cleaner, more vintage-inspired aesthetic that works beautifully for retro-styled Seiko modding projects or builds where dial clarity takes priority over constant seconds display.

 

VK63: Full Tri-Compax Layout

For those who want the complete chronograph experience, the VK63 delivers with its tri-compax configuration. Small running seconds at 6 o'clock, a 60-minute chronograph counter at 9 o'clock, and a 24-hour subdial at 3 o'clock provide comprehensive timing functionality. The date window is positioned at 4:30.

The VK63's layout makes it ideal for racing-inspired builds or Daytona-style homages in Seiko modding. The constant motion of the running seconds subdial adds life to the dial, while the full set of subdials provides all the chronograph functionality serious enthusiasts expect.

 

VK67 and VK61: Specialized Options

The VK67 distinguishes itself by adding a 12-hour chronograph counter to the standard features, making it a true tool chronograph capable of tracking extended time intervals. This makes it perfect for Seiko modding projects aimed at creating professional-grade timing instruments.

The VK61 takes a more minimalist approach with a symmetrical layout: the 60-minute chronograph subdial at 12 o'clock and running seconds at 6 o'clock, with the date at 3 o'clock. By omitting the 24-hour indicator, it achieves a cleaner, more balanced aesthetic that works well for vintage-inspired or dress chronograph builds.

All VK movements share impressive specifications for Seiko modding applications: they're just 5.10mm thick (significantly slimmer than mechanical chronographs), they provide that satisfying mechanical chronograph sweep, and they maintain exceptional accuracy without the maintenance requirements of fully mechanical movements. At their price point, they are an exceptional value for chronograph builds.

 

Beyond Seiko: Miyota's Premium Alternative

While Seiko movements dominate the modding landscape, Citizen’s Miyota has developed movements that offer compelling alternatives for builders seeking specific performance characteristics. Two Miyota calibers in particular have been introduced in the modding community: the 9015 and its successor, the 90S5.

Miyota has built its reputation on providing affordable yet high-quality movements to microbrands worldwide. Their calibers have proven themselves in real-world use, establishing a track record of reliability that makes them comparable with more expensive Swiss alternatives.

 

Miyota 9015: The Benchmark

The Miyota 9015 has earned its reputation the hard way. Since its release in 2009, it has become a go-to movement for microbrands and serious builders who want better performance without jumping into Swiss-level prices. Over time, it has also found a solid place in Seiko modding, especially for builds where thin cases and smoother movement are a major focus on a build.

One of the biggest differences between the 9015 and Seiko’s NH movements is the beat rate. The 9015 runs at 28,800 vibrations per hour, while the NH35 runs at 21,600. This means the seconds hand moves more smoothly on the Miyota, and the movement can keep time more accurately. The 9015 is rated at minus 10 to plus 30 seconds per day, which is a noticeable step up from most NH movements. When properly regulated, many 9015s perform even better than their stated specs.

The 9015 also checks all the boxes you would expect from a modern automatic movement. It has hand-winding, hacking seconds, a date function, and a power reserve of just over 42 hours. The automatic winding system uses a ball-bearing rotor that winds in both directions, which helps keep the watch running even with lighter daily wear.

Where the 9015 really stands out in modding is its thickness. At only 3.9mm tall, it allows for much slimmer case designs than NH movements, which are over 5mm thick. This makes a big difference in dress watches or builds where you want the watch to sit flatter on the wrist. Many case designs simply are not possible with thicker movements, and the 9015 opens those doors.

There are trade-offs. The finishing on the 9015 is plain and functional, not decorative. Some units are also known for rotor noise, especially when compared to Seiko movements. This does not affect performance and can often be reduced with proper lubrication, but it is something builders should be aware of. The 9015 also costs more than NH movements, though it is still far cheaper than Swiss options like the ETA 2824.

 

Miyota 90S5: The Evolution

The Miyota 90S5 builds on the 9015 platform and improves it. It features better shock resistance and overall refinement. Its most noticeable feature is the open-heart design, which exposes the balance wheel on the dial side.

This puts the 90S5 in direct competition with Seiko’s skeleton movements, but with clear advantages. It keeps the higher beat rate, better accuracy, and slim profile of the 9015. For builders who want an open-heart watch without a thick case, the 90S5 is an excellent choice.

Aftermarket support for Miyota movements is still smaller than Seiko’s NH ecosystem, but it continues to grow. For modders willing to plan carefully, Miyota movements offer a strong premium option.

 

Understanding Movement Authenticity and Branding

One of the first concerns new Seiko modders have is whether NH movements are real. This usually comes up after seeing different engravings like SII or TMI and wondering if one is fake. The short answer is simple. Fake NH movements are basically not a thing.

There is no real incentive to counterfeit NH movements. These calibers are mass-produced, easy to source, and relatively inexpensive. Anyone trying to fake them would spend more money copying the movement than they could ever make selling it. Because of that, NH movements are one of the safest components you can buy in Seiko modding. If you receive an NH movement from a normal supplier, it is almost certainly genuine.

You will commonly see two markings on NH movements: SII and TMI. Both are legitimate and both come from Seiko’s manufacturing ecosystem. SII stands for Seiko Instruments Inc., which is based in Japan and has been a core part of Seiko for decades. TMI originally stood for Time Module, a company created to supply movements to third-party brands and manufacturers. Today, TMI is fully owned by Seiko Holdings and operates under Seiko Manufacturing in Hong Kong, while still using the TMI name.

From a builder’s point of view, there is no functional difference between SII and TMI movements. They are made to the same standards, perform the same way, and use the same parts. Neither version comes with papers or certificates, and that is normal. NH movements were never meant to be sold like luxury Swiss calibers with documentation.

Where you do need to be more careful is when dealing with higher-end movements like ETA or Sellita. Those movements are expensive enough that counterfeiting can be profitable, and fake examples do exist. When you move beyond NH movements, buying from trusted suppliers becomes much more important.

For Seiko modding, though, NH movements are about as low-risk as it gets. If the seller is reputable, you can focus on building instead of worrying about authenticity.

 

Choosing the Right Movement for Your Build

With so many movements available, picking the right one for your Seiko mod can feel overwhelming. The key thing to understand is this: there is no single best movement, there is only the movement that fits what you are trying to build. 

Start by deciding what kind of watch you actually want on your wrist. If the goal is a GMT watch for travel or tracking a second time zone, the NH34 is the obvious choice. If you want a chronograph for timing laps, cooking, or daily tasks, the VK mecha-quartz movements are built for that job. If the goal is a slim dress watch that sits low on the wrist, a Miyota 9015 makes far more sense than a thicker NH movement. Function should always come first.

 

Consider Your Aesthetic Goals

Looks matter just as much as features in Seiko modding. Some movements work better for certain styles, and forcing the wrong movement into a design usually leads to compromises.

If you want a display caseback, NH35 and NH36 movements are great options, especially when paired with an aftermarket rotor. They are reliable, familiar, and look good enough for casual viewing. If you want the movement itself to be part of the design, skeleton movements like the NH70, NH71, or the Miyota 90S5 make more sense. These movements turn the watch into a conversation piece instead of just a timekeeper.

For builders who prefer clean and balanced dials, the NH38 is a strong choice. Removing the date window allows for perfect symmetry and a simpler look. On the other end of the spectrum, day-date layouts with the NH36 lean more traditional and tool-focused.

Case design also plays a big role. Slim cases pair better with thinner movements like the Miyota 9015 or 90S5. Chunky dive cases easily handle the thickness of NH movements with no downside at all.

 

Component Availability

Parts availability can make or break a project, especially for beginners. The NH35 and NH36 have the widest aftermarket support by far. There are thousands of compatible cases, dials, hands, crowns, and accessories. This flexibility makes them the safest choices for first builds.

More specialized movements come with more rules. The NH34 needs GMT-specific dials and often extra crystal clearance. VK chronograph movements require cases designed for pushers and specific subdial layouts. Miyota movements work well but have fewer plug-and-play options compared to NH movements.

For a first build, fewer compatibility problems means more time building and less time troubleshooting.

 

The Right Tool for the Job

Seiko modding works because builders have access to a wide range of reliable movements at prices that make experimenting possible. From the ever-popular NH35 to GMT, chronograph, and skeleton options, each movement exists for a reason and supports different types of builds. This variety is what allows modders to create watches that feel personal instead of mass-produced.

In the end, the best movement is the one that matches your goal. Choose carefully, build with intent, and enjoy the process as much as the finished watch.

Happy modding!