Seiko Caliber 8L35

The Seiko caliber 8L35 is an automatic movement that is based on the Grand Seiko caliber 9S55. As with GS movements, the 8L family is said to be assembled by hand at Shizuku-ishi Watch Studio in Morioka. This movement is currently found in several Prospex divers as well as some Brightz Phoenix watches. Generally, dive watches need a solid caseback for increased water resistance, so the 8L35 is rarely seen.

In 2015, Seiko began using MEMS (Microelectromechanical) parts including the pallet fork and escape wheel. At the time of this post (2020), the 8L35 is still found in new Prospex models.

In Seiko’s own words:

“This limited edition is equipped with the high-grade caliber 8L35, which is hand assembled and adjusted by Seiko’s master watchmakers at the Shizukuishi Watch Studio in Morioka. Assembled from 192 parts, it beats at 28,000 vibrations per hour and has a power reserve of 50 hours.” –Seiko

Seiko 8L35 VS Grand Seiko 9S55:

The 8L35 is essentially an undecorated version of the GS caliber 9S55 with a larger diameter balance wheel to handle the torque needed for the larger dive watch hands. The 8L family is not adjusted/regulated to the same standards and specs as a Grand Seiko movement. The 9S55 was announced in 1998 whereas watches with the 8L35 started showing up around 2000.

8L35 / 8L35A / 8L35B / 8L35C:

The general caliber number is 8L35, with 8L35A being the first version. The subsequent letter variations indicate evolution in the movements development.

Accuracy of the 8L35:

Seiko officially claims the accuracy of the caliber 8L35 has a rating of -10/+15 seconds per day in normal temperature conditions (between 5 – 35 degrees C).

Seiko says this about accuracy:

• The accuracy of mechanical watches is indicated by the “daily rate.”
• The accuracy of mechanical watches may not fall within the specifiedrange of time accuracy because loss/gain changes according to theposition of the watch, which is dependent on the conditions of use,such as the length of time during which the watch is worn on the wrist,arm movement, and whether the mainspring is wound up fully or not,etc.
• The loss/gain of mechanical watches is not measured by a daily rate,but by daily rates of one week or so.
• Accuracy variation according to temperature (The parts that compose the accuracy of mechanical watches are made of metals. It is well known thatmetals expand or contract depending on temperatures due to metal properties. This exerts an effect on theaccuracy of the watches. Mechanical watches tend to lose time at high temperatures while they tend togain time at low temperatures.)

In other words, they are telling you not to start freaking out because your watch isn’t within the accuracy range for one day, you will have to monitor your watch’s accuracy over the span of a week or so before you can get a real sense of your timepiece’s performance.

Power Reserve:

The caliber 8L35 holds a power reserve of at least 50 hours. To fully wind the mainspring, turn the crown a minimum of 55 times.

Crown Position Functions:

0 (against the case): Clockwise = Manual Winding / Counterclockwise = Nothing
1 (pulled out one click): Clockwise = Date setting / Counterclockwise = Nothing
2 (pulled out two clicks): Time setting clockwise and counterclockwise

Can you overwind the movement?

No. According to the Seiko documentation, manually winding the movement after it is already fully wound will not break the spring.

When can you adjust the date?

To avoid damaging your watch movement, do not adjust the date between 8:00 PM and 2:00 AM.

How often does the 8L35 need serviced?

According to official Seiko documentation, watches with the caliber 8L35 should be overhauled every 2-3 years. Learn all about Seiko service intervals here. At the time of this post, the starting cost of getting this movement services by Seiko is $398.00 USD.

In Seiko’s own words:

“Depending on the environment where a diver’s watch is used, its inner parts or movementas well as its case or band may be affected. We recommend that you have your watchoverhauled regularly once every 2 or 3 years.”

Brightz Phoenix:

As mentioned above, the caliber 8L35 powers some Seiko Brightz Phoenix watches that are available in other markets around the world (not North America). These watches feature an exhibition style caseback offering a view into the movement.

Here is an image that was posted on Photobucket:

Screenshot of the Brightz Phoenix page from 2012:

Examples of watches with caliber 8L35:

Additional Resources:

• Seiko 8L35 official instruction manual here