Seiko (SII) Caliber NH25A

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. Some of the topics covered in this caliber listing: NH25A Also Known As… Winding the Mainspring Crown Position Functions SII Caliber NH25A Gallery 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. 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. 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. 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.

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