Seiko (SII/TMI) Caliber NH38A Watch Movement

Manufacturer	Seiko (SII, TMI)
Caliber Number	NH38, NH38A
Movement Type	Automatic, self-winding mechanical
Diameter	27.40mm
Casing Diameter	29.36mm
Height	5.32mm
Jewels	Twenty-Four (24)
Vibrations Per Hour	21,600 bph, 3 Hz
Lift Angle	53 degrees
Accuracy Rating	-20~+40 sec/day average
Power Reserve	~41 hours
Rotor Winding Direction	Bi-directional (Seiko Magic Lever)
Manual-Winding?	Yes
Anti-Shock System	Diashock
Hand Count	3
Functions	Central hours; central minutes; central sweeping seconds
Notable Features	True no-date, open balance wheel at 9:00
Hacking Seconds?	Yes
Country of Manufacture	Japan/Malaysia (see below…)
Known Brands	Various Invicta, Gruppo Gamma Venturo Field III (Add your watch in the comments below…)

The Seiko caliber NH38A is a 3-hander “no-date” automatic watch movement with 24 jewels. It is a true no-date without a phantom date crown position. Many watch enthusiasts would agree that that the NH38 is the definition of a solid, no-frills workhorse movement.

The NH38 series has an open balance wheel at 9:00 on the dial side. However, many watch brands do not make use of this design feature and choose to cover it with a solid dial.

Some of the topics covered in this caliber listing:

What is the difference between the NH38A and NH35A?

is very similar to the popular caliber NH35A in which is it based on, with the main difference being that the NH38A is essentially the no-date version of the widely used NH35A/NH36A. Unlike the NH35A, the NH38A is a basic three hand watch and does not have a day or date complication. Another difference is that the NH38A is partially skeletonized with an open balance wheel, viewable from the front side of the watch face.

Note: due to the NH35A being more ubiquitous and easy to obtain, you may find examples of watch brands offering a no-date style watch, but using the the NH35A with a datewheel instead of the NH38A. If you want a true no-date, you should not settle for an NH35A powered watch that simple has the date covered by a solid dial.

Accuracy Rating:

Time Module (TMI) claims an accuracy rating of -20~+40 seconds per day in normal operating conditions.

Japan or Malaysia or…?

The complete Japanese-made caliber number is NH38A3J. For the Malaysia variant, the official number is NH38A3M. Regardless of where the movement was made, these longer numbers are mostly for ordering purposes and not found anywhere on the movement itself. Most are not marked with a country of origin… so, technically, who knows!

Also, despite the official documentation mentioning Japan or Malaysia signed rotors, Caliber Corner has never actually seen one marked with the country of origin from the factory.