Seiko Epson Caliber Ym12a Drawing

S. Epson Corp. Caliber YM12A

Seiko Epson Caliber Ym12a Drawing

Manufacturer S. Epson Corp. (Seiko, Hattori, SII)
Caliber Number YM12A, YM12
Movement Type Quartz, chronograph
Lignes 12”’
Thickness 4.34mm (including the battery)
Battery Cell Number SR927W (395)
Current Consumption
0.80 μ A
Battery Lasts
5 years
Frequency 32,768 Hz
Quartz Type Tuning fork type quartz crystal
Stem 0351584 (0351585 long)
Hand Sizes 1.10mm / .65mm / .20mm x 3
Hands Count 6
Chronograph Style
Hacking Seconds?
Functions Central hours; central minutes; central chronograph seconds (1/5); small running seconds subdial at 9:00; 60 minutes chronograph counter at 6:00; 24 hour indicator at 3:00; quickset date at 4:30
Country of Manufacture Japan
Known Models Timex Q Chronograph (reverse panda dial on bracelet, on strap; cream dial), Timex Waterbury Dive Chronograph (RGB dial, black dial) (Add your watch to the comments below…)

The S. Epson caliber YM12 (YM12A) is a 0 jewels quartz chronograph movement that is made in Japan. It is found in non-Seiko branded watches (such as the Timex Q Chronograph released October 2022). Technical documentation for this movement were found dated as far back as July 31, 2014, so it can be found in watches at least that old. The S in S. Epson stands for Seiko.

YM12 VS 7T12:

The caliber YM12A is based on the Seiko caliber 7T12. It may be a possible replacement for the 7T12, however, keep in mind that the YM12 may be slightly taller. Also, the Seiko 7T12 has a better accuracy rating at +/-15 sec/month compared to the YM12 at +/20 sec/month.


S. Epson Corp. claims that the accuracy of the YM12A should be within a range of +/-20 seconds per month when worn on the wrist in normal operating temperatures of -5 degrees C to + 50 degrees C (~41 – 122 F). This movement does not have a regulator to speed up or slow down the timing.

Crown Stem Removal:

To remove the crown and stem in your YM12A powered watch, you will need to pull the crown out one click to date setting position. Then locate the arrow with the text “Push” on the back of the movement near the crown. Gently press the detent button with your tweezers while carefully pulling out the crown/stem assembly.

Resetting the Chronograph to Zero:

After a battery change you may need to reset your chronograph hand to zero. In order to achieve this, follow the steps below:

Seiko Epson Ym12a Reset Chronograph To Zero Instructions

Low Battery Indicator:

The YM12A is equipped with a low battery indicator (aka EOL). When the battery is low on your YM12A powered watch, the small seconds hand at 9:00 will skip a second and tick every 2 seconds. The time will still be accurate, but this is an indication that the battery should be replaced as soon as possible. The battery needed is a cell number SR927SW (same as 395), 1.55 V.

Need a new battery for your watch? Order here (your order will support this site).

Replacement Prices:

At the time of this post, replacement prices for the S. Epson caliber YM12A were found in the range of $13.00 USD to $22.99 USD. Prices may vary depending on wholesale or single order retail.

Additional Resources:

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