Grail Watch Reference

A Reference Library for Watches from Grail Watch

Seiko 7S36

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The Seiko Cal. 7S36 is a mainstream automatic movement with day and date produced by Seiko Instruments Inc. (SII). It was introduced in 1996 for the popular Seiko 5 and diver model lines, replacing the 7002 family in many cases. Cal. 7S26 is very similar to Cal. 7S26 but with 23 jewels instead of 21. It is also quite similar to Cal. 7S35, introduced a decade later, which lacks the day wheel. Cal. 7S36 has been produced in Singapore, Malaysia, and Japan, and remains in production as of 2019.

Cal. 7S36 is commonly found in the Seiko 5 (SNZ), diver (SKZ), and Monster diver watches.

Cal. 7S36A was the original movement, produced from 1996 through October 2006, when Cal. 7S36B 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. 7S36C, the final version. There are many component differences between the A/B and C series movements, including the date disc and jumper, hour wheel, minute wheel and pinion, cannon pinion, oscillating weight, balance cock, ratchet wheel, first wheel, pawl lever, barrel and bridge, center wheel bridge and pinion, yoke spring, setting lever, and clutch wheel.

Seiko 7S26

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The Seiko Cal. 7S26 is a mainstream automatic movement with day and date produced by Seiko Instruments Inc. (SII). It was introduced in 1996 for the popular Seiko 5 and diver model lines, replacing the 7002 family in many cases. Cal. 7S26 is very similar to Cal. 7S36 but with 21 jewels instead of 23. It is also quite similar to Cal. 7S25, introduced a decade later, which lacks the day wheel. Cal. 7S26 has been produced in Singapore, Malaysia, and Japan, and remains in production as of 2019.

Cal. 7S26 was also sold by Time Module Inc. (TMI) as Cal. NH26 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 4.90 mm.

Cal. 7S26A was the original movement, produced from 1996 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. 7S26C, the final version. There are many component differences between the A/B and C series movements, including the date disc and jumper, hour wheel, minute wheel and pinion, cannon pinion, oscillating weight, balance cock, ratchet wheel, first wheel, pawl lever, barrel and bridge, center wheel bridge and pinion, yoke spring, setting lever, and clutch wheel.

Seiko 7S Family

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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 Jewel23 Jewel
Date Only7S25A
7S25B
7S25C
NH25A		7S35A
7S35B
7S35C
7S55A
7S55B
Day-Date7S26A
7S26B
7S26C
NH26A		7S36A
7S36B
7S36C

ETA 2894-2

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Modular chronograph with ETA 2892-2 base.

This is also used by TAG Heuer as their Calibre 17.