Seiko Cal. 4R15A (commonly called just Cal. 4R15) is an automatic movement with date wheel. It was later updated as Cal. 4R15B and is closely related to Cal. 4R16A/4R16B. It is also offered by Time Module Inc. (TMI) as Cal. NH15A with 21 jewels for use by other watch makers. It was the first of a new generation of movements based on the Seiko 7S family and uses a high-spec Spron 510 mainspring as in the related Seiko 6R family for greater than 50 hours running time. Later members of the Seiko 4R family included hacking seconds and hand-winding, but this is not present in the 4R15 or 4R16.
Seiko 4R15 Operation
The movement can be manually wound with the crown in position 1. Rotate the crown clockwise to wind the mainspring.
The date wheel is advanced with the crown in second position (one notch out). Turn the crown counter-clockwise to advance the date wheel by one day.
The time is set with the crown in third position (pulled all the way out). Rotate the crown clockwise to advance the minute and hour hands and counter-clockwise to turn them backward.
Seiko 4R Family Overview
Introduced in 2008, the Seiko Instruments Inc. (SII) 4R family is a mainstream automatic watch movement used in many Seiko models and offered to third parties by Time Module Inc. (TMI). It is an evolution of the Seiko 7S family of movements, produced since 1996, and is related to the higher-end Seiko 6R family.
All movements in the 4R family operate at 21,600 A/h and measure 12 ligne (27.4 mm) in diameter by 5.32 mm in height. Cal. 4R15 and 4R16 have 22 jewels, Cal. 4R35A has 23 jewels, and all other 4R and NH movements have 24 jewels. The NH series movements sold by Time Module Inc. (TMI) are rated as slightly more accurate from the factory, at -20/+40 rather than -35/+45 for the Seiko branded movements, and some differ in the finish, number of jewels, and rated power reserve.
Seiko 4R Movements
There are two branches of the family: The 4R1x movements (4R15, 4R16) came first and have an advanced Spron 510 mainspring but lack hacking and hand-winding. The 4R3x movements (4R35, 4R36, 4R37, 4R38, 4R39, 4R57, 4R71) appeared after 2010 and have a traditional mainspring but add hacking and hand-winding. The 6R family has both improvements.
There are only a few complicated members of the family, with a 24 hour subdial (4R37 and 4R39), or date by pointer subdial and power reserve indicator (4R57).