The Seiko caliber 6R15 is an automatic movement found in many modern Seiko dress watches and divers.
The general caliber number is 6R15, with 6R15A being the first version. The subsequent letter variations indicate evolution stages in the movements development. Early models of the 6R15 (A, B and some early C) were prone to issues that Seiko improved upon over the years. Because of the changes, not all parts are interchangeable between the variations. As of the time of this post, the latest stable version of this movement is the 6R15D (if this changes, please update the comments below).
Note: apparently some early versions of the 6R15C only have 23 jewels compared to the newer ones with 24J.
The caliber 6R15 is the base caliber for the 6R20, 6R21, and 6R24, and 6R27.
What are the differences?
All additional variations above have the power reserve indicator at 9:00 and a date subdial at 6:00. The difference is that the 6R20 and 6R21 have a day of the week subdial at 3:00. The 6R24 has a retrograde day of the week indicator at 3:00 (as well as a retrograde date indicator at 6:00. Which also explains why the 6R24 has 31 jewels instead of 29). The 6R27 has a power reserve indicator and date subdial. The 6R15 only has the hours, minutes, seconds, and date.
Other than complications, another difference between the base 6R15 and the variations is that the 6R15 beats at 21,600 and the other calibers beat at 28,800 with power reserve that drops from ~50 hours to ~45 hours. The jewel count on the 6R15 is 23, while the other calibers have increased jewel counts to accommodate the additional features.
6R15 vs NE15
The caliber 6R15 is basically a Seiko-only version of the NE15. Seiko Instruments (aka SII) sells the NE15 to other manufacturers (such as microbrands) for use in non-Seiko watches.
Seiko claims an accuracy 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.
The caliber 6R15 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 = Nothing / Counterclockwise = Date setting
2 (pulled out two clicks): Time setting clockwise and counterclockwise
To remove the stem, 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:
Caliber 6R15 Drawings:
This video by Seiko examines the anatomy of the caliber 6R15D. The watch in the beginning is a Seiko Presage Karesansui model number SPB067.
To say that the 6R15 is Seiko’s answer to the Swiss workhorse that is the ETA 2824 would be an understatement. Bottom line is that the 6R15 caliber is that GOOD a movement and with the sheer number of ETA-powered watches out there, it is sometimes nice to have something different to stir up the masses.