Seiko Caliber Nh25a

Seiko (SII) Caliber NH25A

Seiko Caliber Nh25a

ManufacturerSeiko
Caliber NumberNH25A (aka NH25)
Movement TypeAutomatic
Lignes
12”’
Diameter27.40mm
Casing Diameter
29.36mm
Height
5.32mm
Jewels21
Vibrations Per Hour
21,600 bph
Lift Angle53 degrees
Shock SystemDiashock
Power Reserve
41 hours
Winding Direction
Bi-directional (Magic Lever)
Hacking?No
Hand-Windable?No
FunctionsHours, minutes, central seconds, date at 3:00
Country of ManufactureJapan/Malaysia
Known Brands
(Too many to list. Add yours in the comments below…)

The Seiko/TMI/SII caliber NH25A is an automatic movement that was originally introduced around 2008. It is an early version of what has evolved into one of the most popular automatic watch movements in the world: caliber NH35A.  Time Module aka Seiko Instruments labels the NH calibers as part of the “Basic” Mechanical Series. They also have NE calibers that are labeled as “Premium” Mechanical Series.

The current market seems to have more watches with the newer NH35A movement than the NH25A, in part due to the fact that the NH35A started to be widely used around the same time that the watch community dramatically expanded with crowdfunded microbrand watches. The NH25A can still be found in entry level/affordable/microbrand watches, but they will most likely be older models. For a complete rundown of the differences between the NH25 and NH35, click here.

NH25A Also Known As…

The Seiko NH25A movement is basically an unbranded version of the Seiko caliber 7S25. for example, the NH36 is an unbranded version of the 4R36 (day + date) found in the Next Generation Orange Monster. More information here.

NH25A VS NH26A:

There is a similar caliber NH26A with the main difference being that the NH25A has a date window at 3:00, whereas the NH26A has a day/date at 3:00.

Accuracy:

In the technical documentation, Time Module claims that the NH25A accuracy range is within -25~+35 seconds per day under normal conditions. They recorded these accuracy measurements without the date complication being active. This shouldn’t have any noticeable bearing on the accuracy, but it’s interesting nonetheless.

Note: When you attempt to get a measurement of your watch’s accuracy, it’s important to make sure your watch is fully wound or the lack of power reserve may skew the test results. TMI states that measurements should be done within 10-60 minutes after the movement is fully wound up.

Winding the Mainspring:

So what does fully wound up mean? The NH25A should be fully wound after about 55 manual turns of the crown.

This movement is also equipped with the Seiko Magic Lever, so shaking the watch side to side should get it going almost immediately.

Crown Position Functions:

0 (against the case): Clockwise = Nothing / Counterclockwise = Nothing
1 (pulled out one click): Clockwise = Date Setting / Counterclockwise = Nothing
2 (pulled out two clicks): Time setting clockwise and counterclockwise

Note from Seiko: Do not set the calendar between 9:00 P.M. and 4:00 A.M. If the setting of the calendar is made during this period, the day or date will not change to the next day or date. Please set the calendar after changing the time other than the above period.

Rotor Variations:

While many watch brands that use the NH25A will opt for a custom rotor (oscillating weight), as of the official tech sheets from March 2008, two stock rotors are available depending on the country of origin: Japan or Malaysia.

Seiko Nh25 Rotor Differences

Replacement Prices:

The NH25A is mostly unavailable because it has been replaced by the NH35A. You may be able to find some new old stock movements, otherwise you should look into using the NH35A if possible. At the time of this post, replacement NH35A movements were found online for around $46.00 USD. Buy here


SII Caliber NH25A Gallery:

The example of caliber NH25A pictured below was found in this Invicta Pro Diver model 6930.

Sii Caliber Nh25

Invicta Pro Diver Nh25a

Video Action:


Additional Resources:

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