Seiko Caliber 8L55

Manufacturer	Seiko
Caliber Number	8L55, 8L55A
Movement Type	Automatic
In-House?	Yes
Diameter	(needs confirmed)
Casing Diameter	(needs confirmed)
Height	(needs confirmed)
Jewels	37
Vibrations Per Hour	36,000 bph
Lift Angle	53 degrees
Shock System?	Diashock
Mainspring	SPRON
Power Reserve	~55 hours
Rotor Direction	Bi-directional (Magic Lever)
Hacking?	Yes
Hand-Windable?	Yes
Functions	Hours, minutes, central second hand, date at 3:00 (or 4:30)
Country of Manufacture	Japan
Known Models	Prospex models: SLA025J1, SLA037J1, SLA039J1 (Add more in the comments below)

Seiko caliber 8L55 is a high-beat automatic movement that is based on the Grand Seiko caliber 9S85. 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. Generally, dive watches need a solid caseback for increased water resistance, so the 8L55 is usually covered up.

In 2015, Seiko began using MEMS (Microelectromechanical) parts including the pallet fork and escape wheel. At the time of this post (2020), the 8L55 is still found in new Prospex models.

In Seiko’s own words:

“This re-creation incorporates a Caliber 8L55 10-beat movement, specially optimized and adapted for use in a diver’s watch with this structure. This caliber beats at 36,000 vibrations per hour and has a power reserve of 55 hours, and it is also assembled by Seiko’s leading craftsmen at the Shizukuishi Watch studio.” –Seiko

Seiko 8L55 VS Grand Seiko 9S85:

The 8L55 is essentially an undecorated version of the GS caliber 9S85. The 8L family is also not adjusted/regulated to the same standards and specs as a Grand Seiko movement.

8L55 / 8L55A / 8L55B / 8L55C:

The general caliber number is 8L55, with 8L55A being the first version. The subsequent letter variations indicate evolution in the movements development.

Accuracy of the 8L55:

Seiko officially claims the accuracy of the caliber 8L55 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 8L55 holds a power reserve of at least 55 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 8L55 need serviced?

According to official Seiko documentation, watches with the caliber 8L55 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.”

Additional Resources:

• Seiko 8L55 official instruction manual here