Seiko Caliber 6R20

The Seiko caliber 6R20 is an in-house automatic movement with a power reserve indicator and day-date subdials style calendar complication. The movement is part of Seiko’s 6R family of movements and is made in Japan.

6R20/6R20A/6R20B/6R20C:

The general caliber number is 6R20, with 6R20A being the first version. The subsequent letter variations indicate evolution stages in the movements development. Early models of the 6R20 At the time of this post, the latest and current version of this movement is the 6R20A (if this changes, please update the comments below).

Base Caliber:

The caliber 6R15 is the base caliber for 6R series movements such as the 6R20 discussed here (as well as the 6R21, and 6R24, and 6R27). Interesting note: The calibre 6R20 shares the same instruction manual as the 6R21, 6R24 and 6R27 movements.

6R20 VS 6R15:

Although based on the 6R15 framework, the 6R20 reads much differently on paper – and on the wrist. A few of the differences are outlined in the chart below:

6R20 VS 6R21:

There is a similar caliber 6R21 with the main difference being the power reserve indicator display style. On the 6R20, the PR indicator spans from 8:00 (empty) to 11:00 (full), with the indicator hand being closer to the edge of the dial. On the 6R21, the PR indicator spans from around 7:00 (full) to 11:00 (empty), with the pointer hand positioned closer to the center of the dial.

In Seiko’s own words, they describe the placement on the 6R20 as being “center aspect” and the 6R21 as “9 o’clock aspect”. Also note: the span locations may vary depending on model.

Accuracy:

Seiko claims that the cal. 6R20 has an average 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.

Power Reserve:

The caliber 6R20 holds a power reserve of at least 45 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 = Day of the week setting / Counterclockwise = Date setting
2 (pulled out two clicks): Time setting clockwise and counterclockwise

Stem Removal:

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:

Macro Images:

Below is a close-up view of the 6R20 Dia-shock anti-shock protection device, as well as the Etachron regulator, balance wheel and balance spring.

Examples of watches with this caliber:

The 6R20 shown here was found in this Seiko Premier SPB001 watch with dial reference 6R20-00A2, and case reference 6R20-00A0. The caseback is also marked with “KG” and “A4” in a box.

Caliber Commentary:

Watchhound posted a review of the SPB001 (pictured above) here:

“I have worn this watch for about a month now, and it is as good as the watch forums say, in quality and appearance. And, there was a slight “break-in” period where the watch ran a little fast during the day or when resting face-up, and slower when on its side. But now, it stays within a second or two a day, sometimes better! It seems to perform like a quartz watch. It is truly amazing, you will want to wear this watch all the time, to keep it running. Also at night, you can hear it tick with a slight ring or ting, it sounds very well-made, and the fast beats (8 per second) are sort of “intense” sounding. Great watch, I had bought an ESQ automatic which looked great, but ran horribly (even after getting the movement replaced under warranty). Seiko has it down pat.“

Additional Resources:

• List of Seiko watch movements
• Seiko watch service intervals and prices
• Official Seiko instruction manual (pdf)