Seiko Caliber 5M42A Watch Movement

Manufacturer	Seiko Time Corp.
Caliber Number	5M42, 5M42A
Movement Type	Kinetic
In-House	Yes
Diameter	27.6mm
Casing Diameter	27mm
Height	4.3mm
Jewels	6
Frequency	32,768 Hz
Power Supply	Lithium rechargeable battery (watch capacitor)
Capacitor/Battery Part #	30235MZ (3023 5MZ, 3023.55MZ) or 30235MY (3023 5MY, 3023.5MY)
Power Reserve	6 months on a full charge
Accuracy	Less than +/-15 sec/month
Regulator	None
Driving System	Step motor with load compensated driving pulse
Hand Count	3
Functions	Central hours; central minutes; central seconds; date at 3:00 (possibly different location depending on the dial design)
Features	Power reserve indicator via button at 2:00, low battery warning, overcharge prevention
Country of Manufacture	Made in Japan
Known Models	(Add your watch to the comments below…)

Seiko caliber 5M42 (aka 5M42A) is a kinetic movement with a date. This movement is made in Japan.

Some of the topics covered in this caliber listing:

Power Reserve Indicator:

The power reserve indicator on the cal. 5M42 is not a constant indicator. It requires the wearer to press the button located at 2:00 on the side of the watch.

The power reserve indication is displayed via the seconds hand. Seiko recommends pressing the button when the seconds hand is at 12:00. Some watches will have a power reserve scale printed on the dial, spanning from 12:00 to 6:00 (30 seconds) – by pressing the button when the seconds hand is at 12:00, you can use the scale on the dial more accurately.

Below is a chart with the indications for remaining power reserve:

Low Battery Indicator:

The 5M42 is equipped with a low battery/low power indicator, which presents itself by skipping the seconds hand. The hand will jump every two seconds to let the wearer know the battery is low. This is a 24 hour warning and it means the battery needs charged as soon as possible. To do this, swing the watch back and forth to allow the rotor to wind power into the capacitor.

If you haven’t worn your watch for a long time and it won’t hold a charge no matter how much you swing it or wear it, then it possible the capacitor (rechargeable battery) needs replaced. Although Seiko says these batteries do not require any periodical replacement, they actually do stop working eventually, especially if you haven’t charged the watch for an extended period of time (years).

Replacement Capacitor:

The capacitor battery part number appears to be 30235MY, replacing the discontinued 30235MZ part number. You may still be able to find the original 30235MZ here.

As with all watch capacitors, this is a friendly reminder that the number engraved on the battery (TC920S for example) is not the caliber-specific battery part number. The Seiko part number has to do with the design and positioning of the metal tabs welded to the back of the battery cell. Learn more here.

5M42 VS 5M43:

There is a similar caliber 5M43A, with the main difference being that the 5M43 has a day-date calendar complication whereas the 5M42 just has a date. Most of the parts are interchangeable, except for those related to the calendar system.

Additional Resources: