The Seiko 7S family was a widely-produced automatic watch movement produced from 1996 to the present. The NH family is also produced by Seiko Instruments (SII) but sold by Time Module (TMI) to third-party watch makers. The 7S family was the basis for the following 4R and 6R families.
The Seiko 7S was developed for reliability, economy, and ease of maintenance. It features a simplified design to reduce component count and cost, and some components are made of plastic to reduce the need for lubrication. Although it lacks hacking and hand-winding, it has bi-directional automatic winding thanks to the Seiko Magic Lever system. It oscillates at 21,600 A/h and returns respectable accuracy, though Seiko did not often specify the exact number. 7S movements were available in both 21- and 23-jewel versions, were revised many times (resulting in “A”, “B”, and even “C” variants), and were produced in factories in Singapore, Malaysia, and Japan.
The “B” series movements, introduced in October 2006, feature a new balance staff with open-ended stud carriers. The “C” series of about 2011 was more substantially revised, with over a dozen parts changes incompatible with previous movements.
The date-only 7S25 (and similar 7S35 and 7S55) are considerably rarer than the day-date 7S26 (and similar 7S36). Some date-only or time-only watches in fact use the day-date movement because it is so much more common, and the entire family is sometimes referred to as “7S26” due to its ubiquity.
TMI produced 7S movements as the NH25 and NH26 for third-party watch manufacturers. These were based on the updated 21-jewel 7S25B and 7S26B.
| 21 Jewel | 23 Jewel | |
| Date Only | 7S25A 7S25B 7S25C NH25A | 7S35A 7S35B 7S35C 7S55A 7S55B |
| Day-Date | 7S26A 7S26B 7S26C NH26A | 7S36A 7S36B 7S36C |
