Seiko Caliber NH15A Watch Movement

Manufacturer	Seiko Instruments Inc. (SII) / Time Module Inc. (TMI)
Caliber Number	NH15A (aka NH15)
Movement Type	Automatic, self-winding mechanical
Lignes	10.5”’ (10 1/2”’)
Diameter	23.8mm
Casing Diameter	23.4mm
Height	6.01mm
Jewels	21
Beat Rate	21,600 bph, 3Hz
Lift Angle	52 degrees
Ideal Amplitude	Above 150 degrees
Power Reserve	At least 40 hours
Rotor Winding Direction	Bi-directional (Magic Lever)
Hand-Windable?	Yes
Manual Winding Direction	Clockwise
Stem Part #	0351-420
Shock System	Diashock
Hand Count	3
Hands	1.10mm / .65mm / .21mm
Functions	Central hours; central minutes; central sweeping seconds; date calendar at 3:00
Hacking Seconds?	Yes
Quickset Date?	Yes
Country of Manufacture	Made in Japan or Malaysia
Known Brands	(Add your watches in the comments below…)

The Seiko caliber NH15A (TMI/SII/S. Epson/Hattori) is a midsize automatic movement with handwinding and hacking ability. Measuring 23.8mm in diameter, the NH15 is smaller than the standard modern movement, making it ideal for smaller watch cases.

The NH15 was in production since at least 2009. As of this post, is not currently listed as a current offering and may no longer be available.

Some of the topics covered in this caliber listing:

NH15A VS NH15B:

The trailing letters on Seiko calibers refer to the evolution of the caliber. At the time of this post, there is only the NH15A.

NH15 VS NH05:

The NH15 is essentially the calibre NH05 with a larger mainplate and rotor, which creates a larger overall diameter of the movement:

NH15A	NH05A
10.5”’ (23.8mm diameter)	7.75”’ (17.5mm diameter)

NH15 VS NH16:

There is a similar caliber NH16 with the same measurements and other specs, with the main difference being that the NH15 has a date and the NH16 has a day-date.

Accuracy Rating:

Time Module documentation claims the NH05A runs at an average accuracy of -35 ~ +55 seconds per day in normal operating conditions and temperatures.

Power Reserve:

Although the official documentation for the NH15 states a power reserve of “more than 40 hours”.

The mainspring of this caliber is fully wound after 55 turns of the crown. You cannot over-wind this movement.

Crown/Stem Removal:

The NH15 is a small calibre, making it a bit more challenging to navigate around the architecture of the movement. With the crown and stem to the right, there will be a small hole with a button just above it near the edge of the movement. This is the setting lever axle, creating a detent button which will put opposing pressure on the setting lever, allowing for extraction of the stem. The “button” is very small and will require fine tip tweezers or other tool to fit properly.