Seiko Caliber 6r31a Castle In The Sky

Seiko Caliber 6R31

Seiko Caliber 6r31a Castle In The Sky

Manufacturer Seiko
Caliber Number 6R31, 6R31A
Movement Type Automatic
Diameter 27.40mm (needs confirmed)
Casing Diameter
27mm (needs confirmed)
Height 5.25mm (needs confirmed)
Jewels 24
Vibrations Per Hour
21,600 bph
Lift Angle
53 degrees
Shock System? Diashock
Mainspring Spron 510 (needs confirmed)
Power Reserve
~70 hours
Rotor Direction
Bi-directional (Magic Lever)
Hacking? Yes
Hand-Windable? Yes
Magnetic Resistance
4,800 A/m (60 gauss)
Hand Count
Functions Central hours, central minutes, central second hand
Country of Manufacture Japan
Known Models
King Seiko: SPB279, SPB281, SPB283, SPB285, SPB287, SPB291; Presage: SPB215, SPB319, SPB293; Ladies Presage: SPB233, SPB234, SPB236 (Add your watch in the comments below…)

The Seiko caliber 6R31 is an automatic movement that is a family member of what Seiko refers to as the high performance 6R series calibers.

Caliber 6R31 VS 6R35:

The 6R31 is essentially the 3-hand time-only no-date version of the popular caliber 6R35A.

Official Announcement:

The 6R31 was announced in 2021 and was featured in an official Seiko press-release (June 21, 2021) for the Presage Studio Ghibli Castle In The Sky Collaboration Limited Edition watch (ref: SPB215) stating:

A new version of the 6R35 movement joins the 6R Caliber series. The watch is powered by a new version of Seiko’s high-performance 6R35 movement, Caliber 6R31. Like Caliber 6R35, the movement delivers a power reserve of 70 hours and shares the same performance attributes, but without a date window.”

6R31 / 6R31A / 6R31B / 6R31C:

This is still a rather new movement, so currently there is only the 6R31A. The general caliber number is 6R31, with 6R31A being the first version. The subsequent letter variations indicate evolution stages in the movements development. This post will be updated if the movement gets any upgrades.

Accuracy of the 6R31:

Seiko claims the accuracy of the caliber 6R31 has a rating of -15/+25 seconds per day in normal temperature conditions (between 5 – 35 degrees C).

Seiko says this about accuracy:

  • The accuracy of mechanical watches may not fall within the specified range of time accuracy because of loss/gain changes due to the conditions of use, such as the length of time during which the watch is worn on the wrist, arm movement, whether the mainspring is wound up fully or not, etc.
  • The key components in mechanical watches are made of metals which expand or contract depending on temperatures due to metal properties. This exerts an effect on the accuracy of the watches. Mechanical watches tend to lose time at high temperatures while they tend to gain time at low temperatures.
  • In order to improve accuracy, it is important to regularly supply energy to the balance that controls the speed of the gears. The driving force of the mainspring that powers mechanical watches varies between when it is fully wound and immediately before it is unwound. As the mainspring unwinds, the force weakens.
  • Relatively steady accuracy can be obtained by wearing the watch on the wrist frequently for the self-winding type and winding up the mainspring fully everyday at a fixed time to move it regularly for the wind-up mechanical type.
  • When affected by external strong magnetism, a mechanical watch may loss/gain time temporarily. The parts of the watch may become magnetized depending on the extent of the effect. In such a case, consult the retailer from whom the watch was purchased since the watch requires repair, including demagnetizing.

Power Reserve:

The caliber 6R31 holds a power reserve of at least 70 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): 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.

Stem Removal:

To remove the stem on your 6R31 powered watch, make sure the crown is in position 0 or normal position (not date or time setting position). There is a lever with a small indentation for your tool. Gently press down while pulling the stem out. See official instructions below for more guidance:

Seiko Caliber 4r36 Stem Removal

How often does the Seiko caliber 6R31 need serviced?

According to official Seiko documentation, watches with the caliber 6R31 should be serviced every 2-3 years. Learn all about Seiko service intervals here. At the time of this post, the starting cost of getting this movement overhauled by Seiko is $260.00 USD.

Additional Resources:

  • Seiko 6R31 official instruction manual here
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Edward Bakker
Edward Bakker
11 months ago

My King Seiko has this R31 movement. I’ve had in on the wrist for 3 weeks and can report that it’s accurate to about 30 seconds per week. It’s a clearly well regulated specimen. The stated -15/+25 spd is clearly the worst case performance of an unregulated movement.

Edward Bakker
Edward Bakker
9 months ago
Reply to  Edward Bakker

Just a short update on my KS 6R31, based on my experience of it over the past 3 months. Compared to my two other watches, Oris Pro-pilot (Selitta 200) and Oris pointer day (ETA2824), the 6R31 seems to be more accurate. Over a period of 4 weeks it has lost approximately 1.5 minute.
The ETA 2824 has an unregulated variance of +/-12 spd to +/1 30spd. So the performance of this celebrated movement is quite similar to the often derided performance of the 6R31.

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