Everything You Need to Know About the Seiko GMT NH34 Movement

Seiko Modders are all asking right now: can you mod the newly released Seiko 5 Sports SSK GMT watches? These entry level GMT models are not even out a month and they’re already making waves in the watch world. And when they released the NH34 movement a few days later, they sent the minds of watch modders around the world into creative mode.

“What mods can I make with this?”

We hope to answer this question and more in this blog post so keep reading!

And if you already have an idea of the GMT mod you want to make, you can check our available parts here. Need more info on what a GMT watch is? Check out this article.

The "SKX Sports Style GMT Series"

The release of a GMT model in the Seiko 5 Sports line is something that few people saw coming, but we’re sure it answered a lot of wishes for a cheap GMT watch/movement. The SKX-inspired line currently has 3 variants:

• SSK001 (Black)
• SSK003 (Black and Blue)
• SSK005 (Black and Orange)

A Trio of GMT Goodness. Source: Monochrome Watches

At first glance, it looks like the SKX007 coming back to life with its iconic and familiar case design and janky-yet-beloved Jubilee bracelet. The SSK case is only half a millimeter wider than its inspiration, and an astonishing 0.1mm thicker even with the addition of the GMT function. 

This is probably the most popular of the three colorways. Source: The Bold Hairspring

The bezel insert is of course updated to feature a 24-hour indicator, which is also seen on the chapter-ring in an inverted style. Adding all the markers on the outside of the dial helps it keep its clean aesthetic, bearing only the hour markers, a date window, the Seiko 5 logo, and an “Automatic GMT” inscription.

Somehow, Seiko was able to update the 4R calibre with the GMT function without changing much of its design and measurements. The function is what you’d call a “caller” or “office” GMT, meaning the first crown position will adjust the date and rotating the other way will set the 24h hand. The second position adjusts the minute hand and hour hand at the same time. It’s a tad less impressive than a true GMT movement although at this price point, you really can’t complain.

Info about the Caliber NH34A/4R34

The newest movement from Seiko/TMI/SII is probably the juicier piece of news for watch modders. Being that it’s a budget caliber that is capable of telling time in another time zone, it exploded in popularity, leaving shelves empty and barren in its first few days after watchmakers and modders snatched up the stock.

The NH34 movement is quite similar to the NH35 in construction and appearance. They are both 27.40mm in diameter (29.36mm with casing) and 5.32mm thick. Both sport 24 jewels for durability, and operate at 21,600hz. Power reserve also stays at around 41 hours. Accuracy on the NH34 is currently rated at +40 / -20 secs per day, according to Caliber Corner.

Source: Time Module Technical Guide

So many similarities! Great! Does this mean the NH34 is basically plug-and-play with existing mod parts for the NH35? Not exactly. The addition of a GMT function means that there are some minor alterations that you need to do to use it with current mod parts.

Not sure yet if you want to build a new watch with this movement? Read this article to help you decide!

Modding with the NH34A

Since the width and thickness of the NH34 is mainly the same as the NH35, it will have no trouble fitting into the cases built for the NH35. You don’t even need a special caseback for it!

What makes it different that calls for the said alteration is the wider and taller hand post to accommodate the 24h hand.

Source: Time Module Technical Guide

With the addition of a GMT hand, the hand post (where the watch hands are installed on) becomes 0.4mm taller which may cause the seconds hand at the top to bump on the crystal. This can be resolved by switching to a double domed crystal, as the extra clearance will give more room for the seconds hand. 

If you want to build a GMT watch but your design calls for a flat crystal, you can now totally do so without having to worry about the seconds hand touching the crystal. Our NMK941 GMT Sub Case has extra clearance for the body to fully accommodate the dimensions of the NH34 without requiring a double domed crystal to pair with it, giving modders more freedom with the crystal choice.

Source: Time Module Technical Guide

The next issue is the width of the GMT hand post requiring a dial with a wider hole diameter. Since most modding dials are currently made to fit NH35s, the hole diameter will be too narrow and may require broaching with a round file. You need to be careful that the hole is an exact fit; too big of a dial hole and you may leave an unsightly gap. Of course, you can also always just use the dial that comes with the SSK watches.

Recently, we have released an NH34-native dial that features a widened center hole to perfectly fit the GMT hand post. It features a Divemaster design, making it the ideal choice for a GMT Sub homage.

What about modding the SSK001 / SSK003 / SSK005?

You may have gotten yourself one of the Seiko SSK GMT watches and already want to change how it looks. Which parts can you actually mod right now?

1. Changing the movement on an SSK watch may not sound like a logical choice at first since it’s one of the highlights of the watch, but if you do want to replace the movement, it is entirely possible to do so. 
2. Just take note that the new SSK models do not accept standard SKX007 casebacks, but it is compatible with NH style casebacks meaning you can thin down your new Seiko 5 GMT with our slim casebacks. Just keep your OEM caseback gasket as aftermarket ones are not verified to be compatible.
3. The Seiko SSK line uses a push-pull crown like the ones on SRPD 5 Sports watches. You can swap it with a 5KX crown.
4. The dial is replaceable with other 28.5mm dials from Seiko or the aftermarket, but as mentioned in the last section, you will have to broach the inner hole to make it 2.88mm wide and make way for the GMT post. For reference, the NH35/36 dial hole is 2.05mm.
5. As for the bracelets, ones that work for the SKX007 should also work for the SSK on paper, although you will have to reuse the OEM skinny spring bars from the SSK as the lug holes on the SSK case cannot accept fat spring bars.
6. SKX sapphire crystals are compatible with the Seiko 5 GMT case, just note that you need SSK gaskets and not SKX crystal gaskets. Also, a double domed crystal is not actually required but highly recommended so you have enough clearance between the hands and the crystal. You can also just use our SSK flat sapphire crystal to upgrade.
7. If you're already thinking of upgrading to a sapphire crystal, you might also want to consider getting one of our "SSK Adaptor Bezels" that makes your SSK001/3/5 compatible with SKX007-sized inserts, greatly increasing your modding options.
8. Hands made for the NH35/36 will also work for the NH34. That said, NH35/36 handsets don't usually include a GMT hand so you'll have to source one separately.

These are the different cross-compatibilities tested with the SSK001, SSK003, and SSK005, and currently available SKX007 parts. If it is not mentioned here, it is best to assume for now that there are currently no parts available for it aside from OEM ones.

Wrap Up

Everyone is excited to get their hands on the new Seiko SSK GMT offerings, as well as their new budget GMT movement, the NH34. The mods are slowly starting to appear in our feed to our amusement and delight, and we’re quite keen to be a part of this new era of modding by releasing our own GMT-compatible parts and of course, having the NH34 on offer.

Need more inspiration for your next GMT mod? Check out our 5 picks for the best NH34 builds this past year.

Do stay tuned to our website, newsletter and social media as we announce more parts for you to mod with!

Were you able to score yourself one of the three new Seiko Automatic GMT watches? Or are you already building yourself a GMT watch from the ground up with pure aftermarket parts? Let us know in the comments below!