| 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: – Current | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 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.
Hand winding is accomplished with the crown in first position, rotating clockwise.
| 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 |
