Ultimate Seiko NH34 GMT Movement Modding Guide

The release of the SKX007 was truly a blessing to watch fans all over the world, giving birth to Seiko modding, and allowing many of us to create our own perfect watch. Seiko’s second biggest gift to us is arguably their release of the SSK001, SSK003, and SSK005 back in 2022. When it dropped, Seiko fans wondered how good it would look on their wrist, while Seiko modders wondered what they could add, subtract, or upgrade on it to make it look even better.

 

Source: @inked_wrist_shot on IG

 

Make no mistake, the original SSK trio was already great when it was launched, but modding is all about taking something great and making it even better (or making another project watch better). Thankfully, modders did not have to use a perfectly good SSK and gut it just for its 4R34 GMT movement, since the unbranded NH34 movement was released alongside it for the use of microbrands and watch modders.

The new NH34 was every bit as reliable as the NH35, which by then was already known as THE workhorse movement to use for Seiko mods. It has the same diameter, same 24-jewel construction, and only a slight bump in thickness as it crams a whole new complication. It’s what’s called a “caller” or “office” GMT and not a true travel GMT, but for the price, it was a game-changer.

 

Source: @watchbuilds on IG

 

Let’s take a quick look at the SSK watches itself before diving deeper into the NH34 movement.

 

Comparing the SKX and SSK Series

This first SSK trio revived the beloved look of the SKX while adding a GMT complication. Each version features a 24-hour bezel and matching chapter ring, while maintaining a clean dial layout with just the essentials. It’s perfect for those who appreciate a sporty yet simple aesthetic.

Both the SKX007 and SSK share near-identical dimensions: 42.5mm in width, 46mm lug-to-lug, 22mm lug width. This makes them equally wearable and easy to style. Design cues like the drilled lugs, curved case, and Jubilee bracelet carry over, though the SSK's bracelet and finishing are notably upgraded. One key downgrade: the water resistance drops from the SKX’s 200m to 100m on the SSK, though it was no problem as these watches are mostly worn as desk divers nowadays (there, we said it).

 

 

The SKX dial is, at a glance, recognizable for diving, while the SSK is for tracking dual time zones. While both use Hardlex crystals, the SSK adds a cyclops magnifier and a 24-hour bezel insert. LumiBrite lume still glows strong on both, and the classic mix of circular and triangular markers is present though it’s noticeably more posh on the SSK.

Seiko modding-wise, the SKX007 remains the GOAT thanks to massive aftermarket support. The SSK series is newer, and modding options are still catching up. But with parts like NH34-compatible dials, crystals, and bezels already rolling out (check our shop for parts!), the SSK is gearing up to be another modding staple.

 

Quick Explainer: Caller vs Traveler GMT

The main difference between a caller GMT like the one in the Seiko SSK001, and a true GMT like in the Grand Seiko SBGN029, lies in how the hour hands operate when setting the time. In a caller GMT, the 24-hour GMT hand is independently adjustable, meaning you use it to track a second time zone, often for calling someone in another region, hence the name "caller." The local hour hand, however, moves in sync with the minute hand and can't be adjusted independently. 

 

Source: @cheetaseikosha on IG

 

On the other hand, a traveler or “true” GMT features an independently adjustable local hour hand that can be moved forward or backward in one-hour increments without stopping the watch or disturbing the GMT hand. This makes it much more convenient for actual travelers, as they can quickly change their local time when crossing time zones, while the GMT hand continues to track home time.

If you’re not exactly a frequent flyer and just want a GMT function for the looks, then the NH34 is perfectly fine for your next Seiko mod!

Seiko Mods with the NH34 GMT Movement

When the NH34 dropped, you can bet the modders were scrambling to get their hands on it, as well as compatible parts, to make their next build. And to their pleasant surprise, there are actually already-available parts that have some degree of compatibility with it, though with a few adjustments to make it all fit nicely. The biggest changes to account for was the taller hand stack which meant double domed crystals are a must, and the wider GMT pinion which meant dial holes need to be widened.

These days, parts meant specifically for the NH34 are getting more and more common. If you’re new and unsure yet on where to start for your GMT build, here are a few recommendations.

 

Cases

Submariner Case

Let’s face it, our fascination for GMT was brought on by Rolex Submariner. It’s one of our most popular cases even before the NH34 rolled out, so it’s only natural that this is one of the first few designs that we had to improve to accommodate Seiko’s GMT surprise. The NMK941 is our first GMT Sub case that will allow you to use flat crystals, when normally you’d have to pair a double dome crystal with an NH34.

BB58 Case

Rolex’s sister brand Tudor also makes beautiful watches, and the most popular one would be the BlackBay 58 which has inspired many a Seiko mod in itself. Our “Fifty-Eight” case also nicely fits the NH34 and has a sleek, no crown guard look so you can create your custom GMT BB58.

GS Dive Case

Want to build a GMT watch that’s more Seiko-inspired? We can recommend our NMK944 case which is inspired by Grand Seiko dive watches. It has a beefy appearance that’s typical of dive watch cases, while also incorporating elegant angles and cuts that are quite reminiscent of GS offerings.

Square-ish Tool Case

They don’t all have to be cases for dive watch builds - we also have more minimalist case options that will suit GMT builds with a function-first profile. For this we can recommend the NMK932 field case. It has a modern, angular design while also blending in curves for a really unique look that can make it pass as a dress case. Since this case has a bezel-less design, you’ll have to have your 24 hour markers on the dial or chapter rings, perfect if you want the GMT function to be a bit more subtle on your Seiko mod.

Dials and Hands

If you have a broaching tool and are confident with filing the hole on a very delicate mod part, then technically any 28.5mm dial can be used for your GMT mod. If you’d rather have a plug and play dial though, we have a few options for you!

 

 

For matching sets, we have dials in Alpine-style, Divemaster-style, and BB58-style that have GMT-specific holes so there’s no need for additional broaching. They also have GMT inscriptions on the lower area, just to add to the ‘authenticity’ aspect of your build. If you’re looking for something a bit more outlandish, we also have a very colorful Decompression timer style dial meant to go together with a Worldtimer style insert for something that looks like the Mido Ocean Star watch. It looks busy, and therein lies the appeal!

You can check out all of these Seiko modding dials on this page.

When it comes to hands, you’re free to choose anything that will complement your dial as the NH34 uses the same size for the hand posts as the NH35. Just remember to get yourself a GMT hand as these are usually sold separately from three-hand sets.

Bezel/Bezel Insert

If you’re not trying to be subtle by keeping your GMT markers on the chapter ring, the bezel insert is usually where you’d find the 24 hour or dual time markers that will make your GMT hand functional. The most popular options are two-tone inserts like Coke (Black/Red), Pepsi (Blue/Red), Batman (Black/Blue), and etc., but you also still have visually simpler and cleaner options.

Want something with a more tool watch vibe? We have fixed GMT bezels that combine the bezel and insert into one part and snaps on directly to the case. There’s the regular version that will match the height of our regular crystals, and we also have slim fixed bezels that will create a much sleeker watch build when paired together with a slim crystal and caseback.

Modding Inspiration For Your NH34 Build

Want to look at what the Seiko mod community made for NH34 build inspiration? We’ve seen hundreds, if not thousands of awe-inspiring GMT mods over the years but here are a few that stand out:

The Unnamed Masterpiece by @bbmod_watches is a full-on showstopper. Dressed in rose gold from case to crown, this build is centered around a bronze dial featuring a laser-engraved world map. A worldtimer bezel insert encircles the chaos with purpose, while the multi-layered dial with its lumed markers, a circular date window, and sleek rose gold hands, gives off luxe globe-trotter vibes. It’s dramatic, meticulous, and still unnamed. Honestly, it deserves one. Drop your suggestions in the comments!

 

 

On the opposite end of the spectrum is the Vintage GMT Cocaloca by @nhdivers35. This mod leans into the GMT’s aviation-era roots with deliberate aging: a faded Coke bezel insert, a patinated Mercedes handset, and a Seiko PADI dial that looks soaked in sea salt. Set in a classic SKX case, the whole piece feels like a barn find from the ‘60s but with modern internals that will let you enjoy this piece for years to come.

 

Source: @svkwatches.ae on IG

 

Rounding out the trio is a more recent favorite: the Alpine Tool Watch GMT built by @svkwatches.ae. Inspired by the Seiko Alpinist and designed for rugged adventures, this build was made simple with a purpose-built case bundle. With a 24-hr bezel and “GMT” dial variants available, all you really need is your preferred colorway, an NH34 movement, and a compatible GMT hand. It’s the perfect setup for modders who want everyday functionality with real outdoor vibes. Also, if you’re not really of the outdoorsy kind, you can swap out the inner compass for a different rotating chapter ring, like a diver style CR or something with 24hr markers for a more subtle Seiko mod GMT.

Interested in more examples? Check out this article.

Taking Care of your NH34 GMT Movement

The Seiko NH line has always been known to be rugged and trustworthy calibres that can take a beating, but like any mechanical marvel, it needs proper handling to keep ticking accurately. Whether you’re building your first dual-time mod or finishing your first dozen, here are some care tips you should always keep in mind.

1. Keep it away from magnets. Even a small magnetic field can mess with your GMT’s timekeeping. The balance spring is especially vulnerable, which could lead to your watch running fast, slow, or even stopping completely. Watch out for magnetic tools, wireless chargers, and even magnetic drawer latches around your workspace.

2. Avoid exposing it to open air. Dust particles are silent killers. Even the tiniest speck can interfere with your movement’s gears. Keep your modding area clean, store your movement in sealed containers when you’re not using them yet, and use a dust blower before casing. It’ll probably bug you too much and force you to open up the case again anyway if you do find dust inside your watch, especially on the inner side of the crystal!

3. Don’t drop it. Sounds obvious, but it happens. NH movements are built tough, but a single drop can throw everything off balance. When outside of a watch, always handle it over a padded surface and double-check that it functions as expected before you commit to a build.

4. Be gentle with screws. When swapping in a rotor (especially fancy ones like our Asanoha), use the right screwdriver and apply just enough torque. Too tight and you risk jamming or damaging the movement; too loose and you’ll hear it rattle, which should not be happening in this age of Seiko modding.

5. Don’t let it dry up. Regular servicing and lubricating every 3 - 5 years keeps the NH34 running smoothly. Friction wears down even the best engines, and your watch movement’s no different. If you’re not confident about oiling your NH movement, we strongly suggest letting a watchmaker do it for you. But if you’d rather learn and do it yourself, here’s a more in-depth guide.

Treat it right, and your GMT build will keep perfect time for years to come.

 

An Alternative GMT Movement: The Miyota 9075

While the NH34 might be the poster child of customizable GMT watch builds, it’s not your only option. Maybe you’re looking to build the absolute slimmest watch you can build with aftermarket parts, or maybe you’re looking to build something that feels more high end on the wrist. Maybe you want a true GMT instead of an office GMT functionality, or maybe you’re an NH veteran who wants to tinker with something new and challenging for their next Seiko mod build. 

If you’re any of the above, the Miyota 9075 is a strong contender.

Coming from Citizen’s high-end 9000 series, the 9075 beats at a higher frequency of 28,800vph, giving the seconds hand a noticeably smoother glide that modders love to show off to their watch buddies. It’s also a “true” GMT, a slightly more accurate calibre out of the box (-10 to 30 seconds a day, compared to the NH movement’s -20 to +20 per day), plus you get a slight bump in power reserve too - around 42 hours. That all sounds good… so why doesn’t everyone just use it?

 

 

Modding compatibility is where things get tricky. Unlike the NH34, which works with standard Seiko dials and hands (with only minor adjustments), the Miyota 9075 requires movement-specific hands and isn’t plug-and-play with most NH cases. It’s also slimmer than the NH34, which sounds great until you realize most aftermarket cases aren’t designed to take advantage of that profile. Workarounds exist like 3D-printed spacers, but these are currently quite hard to find unless you bump into someone who has already done it before.

Then there’s availability. NH34s are everywhere, with tons of replacement parts and easy servicing. The 9075? A lot more challenging to source. You’ll likely need to hunt around or buy from Citizen directly, which can be a dealbreaker for solo modders.

Still, if you’re building a high-end custom piece and want that buttery sweep and true GMT functionality, the Miyota 9075 is a great choice.

 

Wrap Up

The NH34 movement has changed the game for watch modders everywhere. It brings GMT features, once found only in expensive watches, to anyone with the passion to build their own timepiece. This movement is strong, works with many existing parts, and makes it easy to create watches that track two time zones. You may want a GMT watch for the functionality, or you may be in it purely for the looks. Whichever one you are, you’re free to create the perfect custom watch through Seiko modding.

Whether you want to build something that looks like the famous Submeriner or design your own unique creation, the NH34 gives you a solid foundation. More and more parts made specifically for this movement are becoming available every day. Check out our shop's wide selection of NH34-compatible parts to start your next project. Already built a GMT watch? Our three-hander parts collection is just as impressive for your traditional builds. We have everything from cases down to the gaskets so you can check out our catalog and plan your next custom watch.

Happy modding!