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Seiko Sii Caliber Nh72

Seiko (SII) Caliber NH72

Seiko Sii Caliber Nh72

Manufacturer Seiko (aka Seiko Instruments, SII)
Caliber Number NH72 (aka NH72A)
Movement Type Automatic
Lignes
12”’
Diameter
27.4mm
Casing Diameter
27mm
Height
5.32mm thick
Jewels 24
Vibrations Per Hour
21,600 bph (3Hz) (6 beats per second)
Lift Angle 53 degrees
Shock System Diashock
Power Reserve
~41 hours
Winding Direction
Bi-directional (Magic Lever)
Hacking? Yes
Hand-Windable? Yes
Hand Count
3
Functions Central hours, central minutes, central seconds
Country of Manufacture Japan/Malaysia
Known Brands
(add your watch in the comments below…)

The Seiko/TMI/SII caliber NH72 is a skeletonized no-date 3-hander automatic caliber in the NH family of watch movements. More specifically, it is part of the NH7X series of skeleton dial movements, starting with the NH70 (silver) and NH71 (gilt gold tone). Official technical documents for this series were updated in June 2022, also around when this variation was released.

NH72 VS NH70 / NH71:

The notable (and only) characteristic that sets the NH72 apart from the other two NH70 calibers is the new color: Ruthenium Grey. It is like a lightened PVD coating on the entire movement, including the rotor. Before the release of the NH72, there have been some customizers adding color to the NH7 line and this may be Seiko’s answer to that. Perhaps they will see how the market reacts to it and add more colors later.

What is Ruthenium Gray?

From Wikipedia: “Ruthenium is a chemical element with the symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most other chemicals.” That doesn’t mean the movement actually has any Ruthenium in it, but at least now you know what Ruthenium means.

NH7X movements are based on the popular NH3 (NH35A) framework therefore is hackable as well as hand-windable. The dial feet positions are also the same as the other NHXX calibers if you want to switch your watch to a skeleton ND.

NH72A Dial Side:

Seiko Sii Caliber Nh72a Front

NH72 / NH72A / NH72B:

The general calibre number is NH72, from the NH family of movements, with NH72A being the first version. The subsequent letter variations indicate evolution in the movements development. As of this post, only the NH72A exists, but if there are any upgrades or changes to the movement in the future, the letter will change to NH72B, NH72C, and so on.

Accuracy:

In the technical documentation, Time Module claims that the NH72A accuracy range is within -20~+40 seconds per day under normal conditions. They use three measurement positions: Dial up, crown up, crown down.

Note: When you attempt to get a measurement of your watch’s accuracy, it’s important to make sure your watch is fully wound or the lack of power reserve may skew the test results. TMI states that measurements should be done within 10-60 minutes after the movement is fully wound up.

Winding the Mainspring:

So what does fully wound up mean? The NH72A should be fully wound after about 55 manual turns of the crown. This movement is also equipped with the Seiko Magic Lever, so shaking the watch side to side should get it going almost immediately.

Crown Position Functions:

The caliber NH72 has two crown positions (not counting screwing in the crown if your watch is equipped with a screw-down crown):

  • 0 (against the case): Clockwise = Manual Winding / Counterclockwise = Nothing
  • 1 (pulled out one click): Time setting mode

Seiko Caliber Nh71a Instructions

Hacking:

Since the NH72A is based on the NH35A, that means it has hacking functionality. A hackable watch is one that stops the second hand from advancing forward when the crown is pulled out to time setting mode. Seiko officially refers to this hacking function as a “second hand reset” and the hacking mechanism as a “balance stop lever”.

Country of Origin:

Seiko states that the NH72 is made in Japan or Malaysia and the rotor will be marked with the country of origin, however, Seiko still has not gotten back to us with regards to rotors with no country of origin. China based factories selling SII/TMI movements tend to sell them without a country of origin stamp on the rotor. When asking some of the sellers where the movements are made, they respond “China”. This is inconsistent with information provided by Seiko tech sheets where there is no mention of China being a country of origin. Just something to note as you research these calibers. We will add more information about this as it is revealed.

Replacement Prices:

At the time of this post, replacement prices for the caliber NH72A were found online in the range of $95.00 – 132.95 USD.

Additional Resources:

Seagull Caliber St19

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