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Update: 25th Aug 2022 - NH34 GMT Movement is now available.
With the official release of new Seiko GMT watches (i.e. SSK001, SSK003, SSK005) now available, as modders we have a lot of questions about compatibility of modding the new GMT series with current mod parts in the market.
On top of these new releases, Seiko has also quietly released a new automatic GMT movement, the NH34. This will literally set a new scene for GMT enthusiasts to customise their own GMT mods. But will the current cases and mod parts be compatible with this new GMT NH34 movement?
In this article, we will dissemble a Seiko 5 GMT watch, determine the differences from SKX007 mod parts and compatibility. We will also explain the use of NH34 compatibility with current SKX007 mod parts.
**Note: The difference between SSK001, SSK003 and SSK005 is purely their colour profiles.
Before disassembling, one of the notable difference is that SSK001 GMT has a bi-directional rotatable bezel without clicking sounds when turning. The SKX007 and its predecessors are uni-directional with clicking sounds when turning.
Upon removal of the bezel from the case, we noticed that the bezel back is smooth without teeth. There is no bezel click spring in SSK001.
Basically, SKX007 bezels can't be used on GMT series. For those unaware, GMT bi-directional rotation without clicking sounds can easily be replicated on SKX007 parts themselves: Simply install the bezel without the bezel click spring.
COMPATIBILITY: NO
SSK series bezel inserts are 0.3mm wider in diameter compared to SKX007 bezel inserts.
SSK bezel inserts will not go into SKX007 bezels.
However, SKX007 bezel insert can fit into SSK bezels, but it will be a bit loose in the bezel.
COMPATIBILITY: NO
Given the changes in SSK bezel, the case have also been simplified significantly to accommodate a bi-directional rotatable bezel, henceforth, there is no crevice and holes for fitting bezel click spring. Therefore as mentioned earlier, you would not be able to use a SKX007 bezel on SSK cases.
Another structure change is that the crystal gasket now sits on the chapter ring. In other words, you cannot install a crystal without installing a SSK chapter ring. While in the case of SKX007, the crystal gasket sits on a recess area of the inner case independent from a chapter ring. So in an SKX007 mod, chapter ring is optional, but in SSK, it is mandatory.
With these being said, SSK GMT cases are unique.
Significant changes have been made to the chapter ring. It is now wider and taller to the SKX007 chapter ring. They are drastically different in sizes. The SSK chapter ring also serves an additional function - supporting the crystal gasket.
Outer Diameter & Height
SKX007: 30.5mm & 2.3mm
SSK: 32mm & 2.8mm
On top of that, the SSK plastic chapter ring has two round pins at 12H and 6H. In case you have not noticed in the SSK GMT case photo above, there are two inner holes at 12H and 6H. These holes are there to lock the chapter ring and preventing it from accidental rotation. SKX007 chapter ring only has one pin at 12H, furthermore the pin is semicircle instead of a round circle in SSK.
In short, SKX007 and SSK chapter rings are not interchangeable.
COMPATIBILITY: NO
Since the successor of SKX007 was released last year (i.e. SRPD/SRPE series), there were no longer screw down crowns. Likewise in SSK, crowns are not screw-down.
COMPATIBILITY: NO
The crystal diameter remains the same at 31.5mm for both SKX007 and SSK series. Now you can freely swap to sapphire crystals if you see the need for it.
COMPATIBILITY: YES
In SKX007, caseback gaskets sit on a groove in case prior to screwing down the caseback. While in SSK series, similar to SRPD/SRPE series, there is no groove and the gasket are already tighten onto the caseback. Given the structural difference between SKX007 and SSK cases to accommodate the gaskets, which are essential to water resistance, caseback are definitely not interchangeable.
COMPATIBILITY: NO
Dials that normally fits NH35/36/38/7071/72 will not fit on SSK GMT series. The GMT caliber 4R34 requires a much bigger central hole in the dial in order for the dial to pass through and sit on the movement.
Diameter of Central Hole
Standard: 2.1mm
GMT: 2.88mm
**Technical details can be found on Seiko official listing.
COMPATIBILITY: NO
Fortunately, Seiko has made it their hands fitting dimensions to be same across all their mechanical movements. Therefore, the hour, minute and second hands that fit on 7S, 4R, NHxx will also fit for this GMT 4R34 movement.
The only requirement is a new GMT hand for a GMT movement.
COMPATIBILITY: YES
Quick table to identify parts compatibility between the SKX007 and SSK series:
| SKX007 vs SSK SERIES | |
| Parts | Interchangeable / Compatible |
| Bezel | |
| Bezel Insert | |
| Chapter Ring | |
| Crown | |
| Crystal | |
| Caseback | |
| Dial | |
| Hands | |
| Spring Bars | |
With the release of Seiko NH34 GMT movement, modders will be able to customised their own GMT watch, but is it possible to use NH34 movement in the current market SKX007 or SKX013 cases and mod parts?
The answer is not direct. You probably already noticed the compatibility quick table above - every thing is almost incompatible between SKX007 and SSK.
First off, the NH34 movement diameter (29.36mm) and height (5.32mm) is the same with NH35/36/38/70/71/72 movements. That is definitely good news. That means that cases designed for NH35/36 can also fit in NH34.
Now here is the big difference. Notice that NH34 and other NH movements may have the same movement height of 5.32mm. This is the height excluding anything above main plate surface. In other words, it does not including the height from the dial side up.
On the dial side up (H1), the overall wheels height of the hour, minute and seconds have been increased by 0.4mm from 2.26mm to 2.66mm. This 0.4mm is to accommodate the additional GMT hand and although it may not sound like much, but when it comes to watches internal dimensions, 0.1mm may make or break it.
Notice in Seiko's technical drawings below that NH34 has a height of 2.76mm between dial and crystal, while other NH movements only requires height of 2.36mm. Most cases in the market are fabricated with Seiko's technical drawings or they are made into a much slimmer case profile. The slimmer the case, the chances of fitting in a behemoth height of a NH34 movement is impossible. Now if you were to fit an 4-hands NH34 movement into a case designed for 3-hands NH movement, chances are your second hands will be brushing onto the crystal either slowing the beat rate or completely stopping the movement.
One of the solution to circumvent this is to use a double domed sapphire.
Here's a further break down of 4-hands NH34 vs 3-hands NH movements. Tolerance between hands in NH34 are slight tighter. Those who do encounter difficulty installing hands in modding 3-hands NH movements, 4-hands NH34 movement represent a significant challenge. **TIP: Invest in a professional hand setting tool and a loupe.
Even if a double domed crystal is used, it is worth noting that the length of the second hand may come extremely close or in contact to the edge of the double down crystal that sloped down onto the case.
Below, we simulated in a technical drawings our GS 36mm Case (double domed crystal) with NH34 movement and second hands that is 12.5mm long.
Second hands is not touching the crystal, but extremely close at 0.27mm. Imagine how small is 1mm, now imagine a quarter of an mm. It is not visible to the naked eye. To accurately install a second hand that is so precise is a technical feat of precision, where I dare say a huge majority will not be able to achieve. Especially so without professional tools.
Aside from this, NH34 movement requires NH34 dials. The standard dials for 3-hands NH movements will not fit. Details described in the above Dial section.
So is it possible to use SKX007 or SKX013 mod parts to mod a GMT watch with NH34 movement? It is technically possible with the use of double domed crystal. But it will be a challenge of hand installing skillset and finesse. It may work for some modders and it will not work for some.
Quick table to identify parts compatibility between the non GMT-specific SKX Mod Parts and NH34:
| Non GMT-specific SKX Mod Parts with NH34 | ||
| Second Hands Length | Flat Crystal | Double Domed Crystal |
| 12mm | ||
| 12.5mm | ||
| 12.75mm | ||
| Cap | ||
| **These explanations are assuming the dimensions of cases are OEM design. | ||
In fact, the best bet now is to wait for NH34 GMT-specific mod parts, i.e. a slightly taller case, dials and hands. At the moment, there are a lack of aftermarket NH34 GMT-specific cases, dials and hands. This limits the choice for aftermarket customisation of GMT watches. Fortunately, over the course of a few months, we believe the proliferation of GMT-specific parts would be overwhelming and modders will be spoilt for choice.
So stay tuned as there will be dozens of new NH34 GMT-specific cases and parts coming.
Want to learn more about watch modding? Check out our tutorial posts
