The Seiko Cal. 7S25 is a mainstream automatic movement with date produced by Seiko Instruments Inc. (SII). It was introduced in 2004 for a new line of mechanical Seiko Prospex diver watches, SBDA001, which was produced through 2010. Cal. 7S25 is very similar to Cal. 7S35 and 7S55 but with 21 jewels instead of 23. It is also quite similar to Cal. 7S26, which adds a day wheel. Unusually for the 7S family, the 7S25 was only produced in Japan.
Cal. 7S25 was also sold by Time Module Inc. (TMI) as Cal. NH25 at least from 2008 through 2014. In this form, Seiko specified accuracy of -25/+35 seconds per day and a power reserve of “more than 41 hours”. Seiko also specified total height as 5.32 mm.
Cal. 7S25A was the original movement, produced from about 2005 through October 2006, when Cal. 7S26B was released. These feature updated parts on the balance staff, including an open-ended balance stud. It was further updated by 2011 to become Cal. 7S25C, the final version. This uses a different date disc and jumper, hour wheel, minute wheel, balance cock, ratchet wheel, first wheel, pawl lever, barrel and bridge, center wheel bridge and pinion, yoke spring, setting lever, and clutch wheel than versions A or B.
Seiko 7S26 Applications
Cal. 7S26 is one of the most widely-produced automatic watch movements of all time, appearing in many Seiko 5 models for decades. As of 2020 it is only used in low-end Seiko Automatic models.
- Seiko Automatic SNKP23, SNKP25, SNKP27
- Seiko Automatic SNKN37
- Seiko Automatic SNKN43, SNKN48
- Seiko Automatic SNKM97