| 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).
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