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Brand: | Seiko, Seiko Instruments Inc. | 
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| Family: | |||
| Height: | 5.32 mm | ||
| Jewels: | 24 | ||
| Reserve: | 40, 41 hours | ||
| Frequency: | 21600 A/h | ||
| Winding: | Automatic, Bi-Directional Automatic Winding | ||
| Diameter: | 27.40 mm (12 ligne) | ||
| Complications: | Hacking Seconds, Hand Winding | ||
| Hands: | Central Hour Hand, Central Minute Hand, Central Seconds Hand, Open Heart at 9:00 | ||
| Production: 2012 – Current | |||||||||||
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| 1910s | 1920s | 1930s | 1940s | 1950s | 1960s | 1970s | 1980s | 1990s | 2000s | 2010s | 2020s |
Seiko Cal. 4R38A (commonly called just Cal. 4R38) is a mainstream automatic movement with open heart balance at 9:00. It is also offered by Time Module Inc. (TMI) as Cal. NH38A for use by other watch makers. It is closely related to the date-only Cal. 4R35 and the Cal. 4R37 and 4R39, both of which have a 24 hour subdial at 10:00.
It is also used in the Seiko Original line, where the company specifies accuracy to -35/+45 seconds per day and 41 hours power reserve.
The movement can be manually wound with the crown in position 1. Rotate the crown clockwise to wind the mainspring.
The time is set with the crown in second position (pulled all the way out). Rotate the crown clockwise to advance the minute and hour hands and counter-clockwise to turn them backward.
| Traditional Mainspring Hacking/Hand-Winding | Spron 510 Mainspring | |
| Standard | 4R38A NH38A 4R71A (skeleton) NH70A (skeleton) NH71A (skeleton, gilt) | |
| Date | 4R35A 4R35B NH35A | 4R15A 4R15B NH15A |
| Day and Date | 4R36A NH36A | 4R16A 4R16B NH16A |
| Date, 24 Hour Subdial | 4R37A NH37A | |
| 24 Hour Subdial | 4R39A NH39A | |
| Date by Pointer Power Reserve | 4R57A |
