Seiko Caliber 5206 Watch Movement

Manufacturer	Seiko
Caliber Number	5206, 5206A (Seiko catalog number 52-06-1)
Movement Type	Automatic, self-winding mechanical
Diameter	25.6mm
Height	3.95mm
Jewels	23 or 25
Beat Rate	28,800 bph
Lift Angle	52 degrees
Power Reserve	~40 hours
Rotor Style	Ball-bearing
Hand-Windable?	Yes
Anti-Shock Device	Diachock
Hand Count	3
Functions	Central hours; central minutes; central sweeping seconds; bi-lingual day/date at 3:00
Hacking Seconds?	Yes
Quickset Calendar	Yes
Country of Manufacture	Made in Japan
Known Models	Seiko Lord Matic (Add your watch to the comments below…)

The Seiko caliber 5206 (5206A) is a vintage automatic watch movement in the 52 or 5200 family of calibers. The 5206 was equipped with a day/date calendar complication and is found in Daini Seikosha produced Lord Matic watches from the early 1970s (1970~1973).

Some of the topics covered in this caliber listing:

Different Jewel Counts:

The Seiko calibre 5206A has two variants with different jewel counts: 23 or 25. The main difference appears to be that the 25J version includes an upper hole jewel for the barrel and an upper hole jewel for the 1st reduction wheel. Another difference is the part number for the rotor since the jewel count text is different depending on the number of jewels.

Day/Date System:

The day/date on the 5206 is the instant or “spontaneous” type, meaning it switches over at midnight. Both are “quickset” and the day of the week has a bilingual change-over system.

Seiko 5206 VS 5216:

Around 1972, the cal. 5206 evolved into the Seiko caliber 5216A with a more efficient calendar system, but also eliminated the “spontaneous” instant day/date changeover at midnight. Many of the parts are interchangeable, except for several parts for the day/date system. All this means is that rather than the day/date changing together instantly at 12:00AM, the date will change at 12:00AM and the day of the week will change about an hour or so after that.

With the 5206A system, the manual advancing of the calendar was much more sensitive compared to the updated 5216 (see the safe zone below).

Setting the Calendar:

To set the day/date, pull the crown out one click to calendar setting mode.

To advance the date, turn the crown clockwise.
To advance the days of the week, turn the crown counterclockwise.

To avoid damaging the movement in your 5206 powered watch, Seiko has issued the following precautions for calendar correcting:

Safe zones for setting the date are with the hands set between 3:45AM and 9:00PM.
Safe zones for setting the day of the week are between 7:00AM and 11:00PM.

Interesting side note: when setting the date, the mainspring also gets wound a little. This, of course, does not create an issue with over-winding (as an automatic movement, this caliber cannot be over-wound), but if the same calendar system was used on a manual-wind watch, it could potentially pose a problem with over-winding.