Seiko Caliber 5Y66

Manufacturer	Seiko
Caliber Number	5Y66, 5Y66A
Movement Type	Quartz
Diameter	24mm
Casing Diameter	23.3mm
Height	3.81mm
Jewels	0
Frequency	32,768Hz
Battery Number	371 (SR920SW)
Battery Life	~3 years
Accuracy Rating	+/-20 seconds/month
Driving System	1 piece stepping motor
Stem	351177 (tap 10)
Hands Count	6
Functions	Central hours; central minutes; central seconds; pointer date subdial at 3:00; 24 hour indicator at 6:00; retrograde day of the week between 8:00 and 11:00 (around 9:30)
Hacking Seconds?	Yes
Country of Manufacture	Japan (assembled)
Known Models	(Add your watch to the comments below…)

The Seiko caliber 5Y66 is a quartz day-date movement with no jewels.

The A in 5Y66A:

Your movement may be marked as a 5Y66A. The A indicates the evolution of the movement, with A being the first. Therefore 5Y66 is the same as 5Y66A. As of this post, only 5Y66A exists, not 5Y66B, C, etc.

5Y66 VS 5Y67:

There is a similar Seiko caliber 5Y67. They both share the same instructions manual and technical documentation. The main difference between the two movements is that the 5Y66 has a larger retrograde day of the week at 9:30 (between 8 and 11), and the 5Y67 has a retrograde day of the week at 10:30 (between 9:30 and 11:30).

Additional Resources:

• Official instruction manual (pdf)