Seiko Caliber 7T12

Seiko caliber 7T12 (actually marked 7T12A on the movement) is found in some Seiko branded chronograph watches. This quartz movement is made in Japan and features a 60 minutes chronograph stopwatch with a 24 hour indicator. It is a 0 jewels movement..

7T12 vs YM12:

You may also see a similar caliber known as the Hattori or SII YM12. The YM12 is basically the same movement as the 7T12 except that it is sold to other watch companies and used in non-Seiko branded timepieces. It may be possible to interchange these movements and their parts.

Learn more about Hattori/SII/TMI/Epson here.

Chronograph Function:

The chrono hand on the 7T12 pulsates 5 times per second. This also allows this movement to measure 1/5 of a second when using the chronograph feature. To reset the chronograph hands see below.

Accuracy:

Seiko claims that the caliber 7T12 will maintain accuracy of +/- 15 seconds per month at normal operating temperatures of about 41F to 95F.

Battery Change:

In the official manual, Seiko claims that the battery should last approximately 5 years. Keep in mind that the battery will drain faster if you leave the chronograph second hand running – specifically, if the chrono is used for more than 2 hours per day.

There is also a low battery indicator which will cause the small second hand to jump every 2 seconds. The time on your watch will maintain accuracy even in low battery mode.

The original battery in the watch was also made in Japan by Seiko. The size is SR927W which converts to a 395 cell.

Resetting the Chronograph Hand to Zero:

After a battery change, the chronograph hands on your watch may need recalibrated to get them to line up at zero.

There are only 2 hands related to the chronograph experience on the 7T12A movement: the central chronograph seconds hand and the 60 minutes chronograph totalizer at 6:00.

Note: the 24 hour hand at 3:00 is not able to be set or changed independently. The 24 hour hand follows the main central hour hand and simply indicates the 24 hour time (aka military time). It is more of a day/night indicator or AM/PM indicator.

Here’s how to line up the 2 hands:

• Pull the crown out to time setting mode (2 clicks).
• Press the top chronograph button for 2 seconds. The chrono minutes hand will make a full rotation, it is not in setting mode. Press the bottom chronograph button to advance the hand until it is lined up.
• Press the top chronograph button for 2 seconds again. Now the central chronograph seconds hand will make a full rotation.
• Press the bottom button to advance the hand until it is where you want it.
• Press the crown back in.

Additional Resources:

• Seiko 7T12 official instruction manual (pdf)