The Citizen Miyota 8217 is a robust and accurate automatic movement manufactured by Citizen Miyota in Japan for a few decades now, having had its debut in 1977. The 8217 is one of the strongest, most solid automatic movements on the market.
It is a 21 jewels movement beating at 21,600 bph or 3Hz (6 half-cycles per second). Jewels, typically synthetic rubis are used in watch movements, not to add to their value, but to reduce friction and wear on moving parts. Lower end watches may have only 7 to 15 jewels. The standard for quality watches is 17 jewels or more. Tipically, anything over 25 jewels for these types of automatic movements are just for decoration.
Most modern watches have some form of shock protection around the balance wheel, as that’s one of the most delicate parts of the movement. This key part is the one most likely to break upon impact, which until the 1930’s or so was a real problem. The basic idea is to house the jewel in a circumferential spring mount, so that it can flex and absorb the energy. Citizen Miyota has developed in 1956 a proprietary shock resistance technology called parashock used till this day in many of their movements as in the Miyota 8217.
The auto winding mechanism of the Miyota is unidirectional and charges the spring barrel when rotor moves counter clockwise. The movement has a power reserve of about 42 hours and its daily accuracy is of about -20 to +40 seconds per day. The date setting mechanism also has a quick set mode as found in majority of modern mechanical watch movements.