Seiko Mod Compatibility of NH34A vs NH35A

Can you swap a NH35 with a NH34 in a Seiko Mod?

The addition of the 24 hour hand on a NH34A causes it to be slightly different than its NH35 sibling.

NH34A is the new movement from Seiko with a second hour indication, or better known as GMT function. As we outlined in our previous blog post, the NH34 is a movement with largely the same dimensions. That invites a possibility: is it be possible to use the NH34 as a GMT upgrade to watches with existing Seiko NH movements? Well, we went into the design diagrams of the said movements and compare the numbers. Here is what we found out.

Yes NH34A is compatible with NH35A

Whether the movement fits in existing watches depends on its outer dimensions, namely its diameter and thickness. Good news is, NH34A shares the same outer dimensions with those of NH35. The precise figures can be found in this blog post if you want to verify the numbers yourself.

This is not surprising, as Seiko intends to make this an variant of their mainstay NH35 movement, only adding a second hour module to the said movement. The design of the movement stays largely the same.

But with a couple of small caveats

With the addition of the second hour module, the hand post, or also known as the center post, gets a small adjustment to its dimensions. This affect its mod compatibility in two ways:

Hand post height: basically we are looking at the H1 measurement in the middle. Notice the extra step at the bottom of the post, which cause the hand post to be stand slightly prouder than that of a NH35A.

Notice the extra step at the bottom of the central post. It is the main differentiator from a NH35a.

1. Hand post height: the addition of second hour module means that the hand post is one level higher, as you need to fit one more hand vertically. This amounts to a slight increase in the height of 40c or 0.4mm. This difference in height affects how much clearance the second hand has from the bottom of the crystal. This is tolerable if the watch already has a high clearance between the second hand and the crystal. If that is not the case, you can still get by with a tighter and more precise second hand installation.

2. Dial hole diameter: the addition of a second hour to the center post causes the post to be wider in diameter near the base. This means that your existing dial might not fit. Fortunately, the fix is simple. You can broach the center hole with a round file, working incrementally until you get to a point where the hole fits the post. The difference in diameter for the reference dial is 0.83mm, if that information is of any use to you.

In the center: the width of the NH34A dial hole is 2.88mm. The corresponding measurement of a NH35 is 2.05mm, so the resulting difference is 0.83mm.

Good news: NH34A Seiko Mod is Possible

So there you have it. The NH34A can be swapped with a NH35A after two simple alterations, which basically means that it is possible to mod a Seiko watch with a NH34A. This is great news for Seiko mod enthusiasts, as you can now turn an ordinary Seiko watch into a GMT watch. This is pretty exciting!

If you want to know more about this GMT movement from Seiko, you can check out our detail coverage on its basic specs and measurements here.

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