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Watch Modding FAQ: Working With the New Seiko 5 Sports GMT

chapter rings

Seiko recently launched a new automatic GMT watch and the question on everyone’s mind is – “What new mods can I make with these watches?” Read on to find out!

Is it possible to change the movement? How “moddable” is it?

The big question among modders has been the versatility of the new 4R34 movement and much to everyone’s delight, Seiko has designed the new 4R34 to have approximately the same dimensions as similar movements like the 4R35 and 4R36. The biggest difference is the center post which is a bit taller to accommodate the 4th hand – the GMT. This means you can drop a 4R34 movement in pretty much any case designed for older Seiko models and be okay – with one big caveat. That is, you’ll need a crystal curved enough to give space for the taller post. Scroll down to the section on crystals to learn more!

Will SKX-style chapter rings fit in the new Seiko 5 GMT?

No – this is one piece that has been redesigned for the SSK line of GMT watches and chapter rings design for use in SKX or SRPD cases won’t fit. Part of the reason? As a GMT watch, the chapter ring in the SSK watches plays an important role showing the 24-hour time scale. As such, Seiko redesigned it to be higher-profile, standing taller than its predecessor. Additionally, because  misalignment was a frequent complaint among SKX and SRPD fans, Seiko added a second alignment tab to the new chapter rings. These correspond with an additional slot in the case (but we’ll get to that. Keep reading!)


Do the new Seiko SSK models need a special dial?

Sort of. The new 4R34 movement has a thicker center post (where all the hands attach). This is because the 4R34 needs to accommodate the 3 regular hands (hours, minutes, seconds), plus a GMT hand. Because the diameter of the center post of the new 4R35 is bigger, SSK dials have a slightly larger center hole to accommodate. If you tried to install an SKX or SRPD dial directly on an 4R35 movement, it wouldn’t fit. However, there’s hope! You could (carefully) expand the diameter of the center hole on a dial designed for 4R35 OR 4R 36 and make it fit! As with all watch mods, the secret is to go slow and be careful to avoid damaging parts!

Another thing to bear in mind when it comes to dials: the clearance between the GMT hand and the dial. Certain dials with applied indices could cause issues if the GMT hand (which sits closest to the dial surface) bumps into the markers. To avoid this, use dials that are painted or printed.


Does the new SSK line of watches have a click bezel?

No! The new SSK watches use a bi-directional bezel, different from the diver style uni-directional bezels found on other Seiko 5 Sports watches. This new design means bezels designed for SKX or SRPD watches won’t fit on the SSK case. Conversely, SSK bezels won’t fit on those cases. Here, it all comes down to case design. Because the case and bezel are so unified in their design, one must be paired with the other. That is, you must match SSK cases to SSK bezels.

What crown is compatible with the Seiko 5 GMT?

Good news! Crowns designed for the Seiko 5 Sports line will work with the new GMT watches! These push-pull crowns are reliable thanks to quality gaskets and great grip. Seiko fans will remember that when Seiko introduced the 5 Sports line to replace the SKX, one big change was the switch from screw-down crowns to push-pull style. As such, “SKX” style cases always use screw-down crowns and “SRPD” style ones use push-pull crowns. The new SSK watches fall into the latter camp and their cases won’t accept screw-down crowns.


Can I change the hands on the SSK?

Yep! The 4R34 movement is compatible with hands designed for all sorts of famous Seiko movements. In fact the hour, minute, and seconds hands of the 4R35, 4R36, 7S26, 7S36, and others are all compatible with the new 4R34. When it comes to the GMT hand, there’s fewer options available but more aftermarket styles arriving every day.


How is the crystal different on the Seiko 5 GMT? Can I swap crystals?

Yes – you can swap crystals. The diameter of the crystal is the same as SKX and SRPD models (31.5mm) but the thickness is different. That is, crystals on the new SSK models are slightly thinner than before. Part of the reason? Since the center post of the new 4R34 movement is taller, there needs to be more clearance between the top of the post and the underside of the crystal. One way to do this is via case design. The other is by making the crystal thinner! Does this mean you can’t use the new 4r34 GMT movement in old-style SKX or SRPD cases? Nope! The movement will fit, you’ll just need to use a crystal with a convex shape on the underside. These are often referred to as double-dome crystals because both the top and the bottom are shaped like a dome. Originally, this was to prevent distortion but when paired with a 4R34 movement, it has the added benefit of giving enough room for the taller center post to fit!

bezel insert

Can you use an SKX007/SRPD bezel insert on the Seiko 5 GMT?

Not really. But that doesn’t mean you can’t get creative! For quick reference, use the list below:

  • SSK bezel + SSK bezel insert: Compatible, by default
  • SSK bezel + SKX/SRPD bezel insert: Not compatible.
  • SKX/SRPD bezel + SSK bezel insert: Not compatible.
  • SKX/SRPD bezel + SKX/SRPD bezel insert: Compatible, by default

Overall, your best bet is to choose a style (SSK or SKX/SRPD) for the case, bezel, and insert and stick with it!


What straps or bracelets are compatible with the new SSK?

Pretty much everything that’s 22mm! This is unchanged from previous SRPD models so anything designed for that case will fit the SSK. Additionally, countless aftermarket straps that are 22mm will fit and make your watch look great! Note: you’ll need “skinny” springbars since the SSK has different diameter lug holes. Hint: just use the ones that came with the SSK!

Since 1913, Feldmar has been repairing watches so you could say modding is in our DNA! Feldmar is also an authorized dealer for over 60 brands including Seiko, Citizen, G-Shock, and more so you can pick up your new Seiko 5 Sports GMT watch.

While we don’t sell mod parts, we are excited to partner with our neighbors at Crystal Times USA. They are a terrific place to start your mod journey! Are you interested in modding or have you modded a watch in the past? We’d love to see it! 

About The Author: Tom Roth


Born in Washington state, Tom developed an interest in photography during college at University of San Diego. There, he got started in music journalism, interviewing artists and taking photos at concerts. A life-long tinkerer and collector, it wasn’t long before Tom became fascinated with fountain pens and watches.

Those interests collided in November 2020 when Tom started at Feldmar Watch Company where he lives out every watch geek’s dream: photographing and writing about timepieces. When he’s not tinkering with his watch collection, Tom can be found traveling, biting his nails while watching PNW sports teams, and taking flying lessons.

Read more from Tom Roth

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