Seiko Caliber 5s21a

Seiko Caliber 5S21A

Seiko Caliber 5s21a

Manufacturer Seiko
Caliber Number 5S21, 5S21A
Movement Type Quartz
Diameter 24mm
Casing Diameter
2.2mm (2.3mm with battery)
Battery Cell
SR920SW (371)
Battery Life
~2 years
Battery Voltage
Driving System
.25 seconds step motor (load compensated)
Accuracy Rating
+/- 20 seconds per month
Hand Count 3
Functions Central hours; central minutes; central sweeping seconds
Hacking Seconds?
Country of Manufacture Japan
Known Models
Seiko Spirit, Seiko Dolce (Add your watch to the comments below…)

The Seiko caliber 5S21A is a quartz movement with a sweeping seconds hand. This movement was introduced around 1988/1989 and is no longer being produced. Although the shape of this movement is rectangular, it can be found in rectangular (Dolce) or round (Spirit) shaped watches.

Seiko 5s21 Drawings

Sweep Seconds Quartz:

Similar to what you might find in a high frequency Bulova Precisionist or mechaquartz movement, the 5S21 has a smooth sweeping seconds hand rather than the 1 second “ticking” that most watch enthusiasts are used to. As you can see from the videos below, with a 5S21 powered watch on the wrist, most people would think it is a mechanical watch.

How does it work?

The excerpt below was found posted on a Japanese site that collects and publishes papers on tech from Japan. It explains, presumably in Seiko’s own words, how it works:

“This paper describes a new quartz watch driving system we developed. With this system, the second hand of a wrist watch moves smoothly as if it is sweeping. An intermittent movement of the stepping motor is transferred to a smooth sweeping motion due to the spiral spring and the viscous regulator. This system also allows the continued superiority of quartz watches.” –Source

Similar Calibers:

There is a similar caliber 5S42A. One difference is that the 5S42A has a date at 3:00 with two crown setting positions and the 5S21A is a true no-date movement with one crown setting position. Also, upgrades were made to how the hacking mechanism works. Upon pushing the crown back in after setting the time on a 5S21A powered watch, the seconds hand may advance forward at a faster rate until the stored energy is released. In the 5S42A, hacking functions normally and the seconds hand simply starts off at a normal rate from where you hacked it.

Smooth Seconds in Action:

5S21 VS Spring Drive:

Below is an interesting video comparing the Seiko 5S21A quartz with a caliber 7R88A Spring Drive movement. Although Seiko was working on Spring Drive technology since the early 80s (1982), they didn’t officially present it until Baselworld 1998. The following year (1999) and about 10 years after the 5S21A was released, they made some limited edition pieces available to the Japanese domestic market. Did they stop production of the 5S21 because of the move towards Spring Drive? Was the 5S21 a product of their prototyping? Technically the 5S21 does have a hair spring wheel and pinion.

5S21A Disassembly/Assembly Videos:

Seiko caliber 5S21A disassembly:

Seiko caliber 5S21A assembly:

Seiko 5S21A official tech sheets (pdf)

Additional Resources:

Seagull Caliber St19

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