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Seiko SII TMI NH25A vs NH35A Differences Compared

Seiko Caliber NH25A VS. NH35A

Seiko SII TMI NH25A vs NH35A Differences Compared

What is the difference between Seiko caliber NH25 and NH35?

There are some differences between these overall similar movements. The differences are mostly convenience that give value to the collector. Please compare in the chart below. Did we miss anything? Please comment below.

Seiko Caliber NH25A Seiko Caliber NH35A
Seiko Caliber 7S25 4R35
Origin Japan/Malaysia Japan/Malaysia
Year Introduced (Please add to comments below) 2011?
Diameter 12”’ (27.40mm) 12”’ (27.40mm)
Casing Diameter 29.25mm 29.36mm
Height 5.32mm 5.32mm
Jewels 21 24
Power Reserve > 41 Hours > 41 Hours
Beats Per Hour 21,600 vph (6 vibrations per second) 21,600 vph (6 vibrations per second)
Calendar? Yes Yes
Hacking? NO! Yes
Winding Direction? Bi-directional Bi-directional
Hand-Winding? NO! Yes, can also be manually wound
Crown See chart below See chart below
Diashock Diashock
Quick Set Date?
Yes Yes
Models Add yours to the comments below Add yours to the comments below

As you can see in red, the most notable differences between the SII caliber NH25 and NH35 are that the NH25 is non-hacking and non-manual winding. The newer NH35 is a result of Seiko evolving the movement, perhaps as a result of listening to the watch community and observing that these two features are important to watch enthusiasts.

Difference Between Crown Functions

As we see here, the main differences between the two calibers is that the NH35A is hackable and can be hand-wound. That means the crown operates differently:

Crown position 0 Free (does nothing)
  • Clockwise: Manual Winding
  • Counterclockwise: Free (does nothing)
Crown position 1
  • Clockwise: Sets the date (quick set)
  • Counterclockwise: Free (does nothing)
  • Clockwise: Free (does nothing)
  • Counterclockwise: Sets the date (quick set)
Crown position 2 Time setting Time setting, second hand reset

On watches with the NH25A, when the crown is pulled out to time setting position (position 2) the second hand keeps going, it “doesn’t hack”. On watches with the NH35A, when the crown is in time setting position, the second hand stops. In other words, it “hacks”. In the official documentation, Seiko refers to this as Second Hand Reset. In other documentation, they also call it a Stop Second Device. Either way, the NH25 is without it!

Accuracy Differences

Time Module claims that the caliber NH25A is rated for a static accuracy of -25 ~ +35 seconds per day. The caliber NH35A clocks in at a static accuracy of -20 ~ +40 seconds per day under normal temperature conditions and without the calendar function. These measurements were taken within 10-60 minutes of being fully wound.

Does my watch have the NH25A or NH35A movement?

It should say on the rotor. If it has a closed caseback or a custom rotor without any caliber markings, the easiest way to tell is to simply wind the crown (unscrew it first if your watch is a dive watch), is it winding the mechanism? Then it’s an NH35A. Pull the crown out two clicks (to position 2 aka time setting mode), does it stop the second hand? Then it’s the NH35A.

NH25A VS NH35A Drawings

Seiko SII TMI NH25A vs NH35A Drawings

Still don’t believe it?

But I heard that the NH25 is manual winding and hacking!

That is incorrect, usually from folks who are mixing up the NH25A with the NH35A. This sometimes happens from collectors who buy a watch that is listed with outdated or incorrect specs, but not actually being able to tell what the actual movement is due to lack of knowledge or a solid caseback covering it up.

For example, the Invicta Pro Diver has had the NH25A inside, but newer models have the NH35A. The collector can read the incorrect specs, thinking they have an NH25A, then get the watch which is powered by an NH35A, then report erroneous information to the watch community that the NH25A in their Pro Diver hacks and winds manually.

Please allow us to interrupt your gear turning and caliber learning to say thank you. Thank you for being here! Now, back to the movement..

Here is are screenshots of the official TMI tech sheets for the NH25A. It clearly states “There are no manual winding function for Cal. NH25/26.”

Seiko Nh25a Manual Winding

And another one here stating: “Cal. NH25/26 have no manual winding function.”

Seiko Nh25 No Manual Winding

As for hacking, the reason that the NH35A is able to stop the seconds when the crown is pulled out, is thanks to a part called the Balance Stop Lever aka hack lever. The NH35A documentation mentions this part and where to oil it during an overhaul.

Seiko Nh35a Balance Stop Lever

The NH25A documentation makes no mention of the balance stop lever, because it doesn’t have one.

So which one is the better movement?

That is up for debate! We’d love to hear your thoughts in the thread below. Why do you like one over the other? What are the advantages of one over the other?

Additional Resources:

  • Seiko NH35A caliber page and comment thread here
  • Seiko NH25A caliber page and comments here

Also keep in mind that although the NH25 and NH35 calibers are the most talked about, there is actually a slew of other NH calibers: NH05, NH15, NH06, NH16, NH36, NH37, NH38, NH39, NH70, etc

Is the Seiko Caliber NH25A VS. NH35A any good? Share your opinions, experiences and watches below... Keep comments respectful and follow our community guidelines.

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Wow, awesome guide! Exactly what I’ve been searching for. You covered everything. A+


I have the NH25A and NH35A. Love them both. Bookmarked the site. Keep up the great work.


another excellent informative post from the editor of the site.


What does the A stand for?


i think it stands for automatic


Hi. “A” is the revision of the calibre.

Seiko (SII) Caliber NH35A Watch Movement |

[…] Also see this thread: NH25A Vs. NH35A Comparison/Differences […]


You might mention that they both have bidirectional winding

Seiko Caliber NH Variation Differences Watch Movement |

[…] these overall similar movements but they are all based off of the original NH35 caliber which is based off of the NH25. The differences are mostly related to day/date/24 hour indicator. Please compare in the chart […]

ID doesn't matter
ID doesn't matter

Since this is a reference document: 1: This sentence means nothing without a TIME reference: “Time Module claims that both the caliber NH25A and the NH35A are rated for accuracy of -25 ~ +35 under normal temperature conditions”. It’s like saying I’ll see you in 7 or 8, Adios. How about per day, per week per month per year…. NooB’s would appreciate it.
2: Please reference which Seiko branded movements the NH movements mimic, always, i.e. 7S26, 4R35, 6R15 etc. thanks.


Don’t feed the troll.


Precision + 5 to 0 second per day for NH35A but +or- 7 second during sport daily activity . Not note for temperature.

Syndicated Satellite
Syndicated Satellite

I noticed a number of watch stores and sellers will wrongfully list their watches as having an NH35A movement, when it really has an NH25A movement.

This is true! Not only will some sellers list NH25A as NH35A (because they either don’t know the difference or they simply default to the most popular one), but similarly with the ETA 6497 vs 6498, Miyota 8205 vs 8215, and sometimes even the 7750 vs 7753. We’ve also seen some microbrands incorrectly labeling the movements they use. Interestingly, prior to the so-called ETA crisis that pushed a lot of brands to start manufacturing their own movements or find alternative suppliers, calibers weren’t discussed as much. KeepTheTime is one the few watch shops that has always listed the caliber details… Read more »

Miyota 8200 mvt opinions pls Vs Orient "Mako" mvt - Page 2

[…] the NH35A is meant to be better. The NH35 has hacking/handwinding where the NH25 does not. SeeSeiko Caliber NH25A VS. NH35A Watch Movement | Reply With […]


Thanks for answering my questions. My new favorite site. Bookmarked! I thought I had a nh35 because that’s what the manufacturer listed it but it’s actually a nh25. Damni

Frank Giann
Frank Giann

I own over 30 watches with both these movements and they are both great but over the long run the NH25A’s have kept more accurate time. I have this one Invicta F0066 with the NH25 movement. It gains a minute every 60 days. This is absolutely outstanding time keeping and I wear this watch every single day to work.

Invicta's diver fake? - Page 3

[…] hand wind, and it does hack, and does keep very good time. Some erroneous info out there then:Seiko Caliber NH25A VS. NH35A Watch Movement | Reply With […]

Seiko (SII) Caliber NH35A Watch Movement | Caliber Corner

[…] Seiko/TMI/SII caliber NH35A is a hand-windable, hacking upgrade from the Seiko caliber NH25. It is one of the world’s most popular automatic movements and is widely available in many […]