Lock and key combination with changeable combination of locking pieces

A lock and key combination comprised of a casing, a lock cylinder mounted within said casing, and a key for turning said lock cylinder between a locking position and an unlocking position, said lock cylinder comprised of a plurality of locking pieces connected in a stack and retained in a line in a locating groove inside said casing and fastened with a driving element by a clamp, said key comprising a key blade having a plurality of key blade slots, wherein each locking piece comprises:

• a base having a short cylindrical configuration having a front recess, a keyway in a middle of said front recess, a plurality of retainer element receiving chambers and spring receiving chambers perpendicularly connected to the keyway of said base at two opposite sides within said front recess, a top recess on a periphery in communication with said front recess, two opposite side notches on a periphery at two opposite sides, and two opposite retaining holes at a back side thereof respectively connected to said side notches;
• a first spring element mounted within the top recess of said base;
• an actuating plate mounted within the front recess of said base, having a projecting locating plate at a top side thereof perpendicularly inserted into the top recess of said base and forced outwards by said first spring element, an elongated slot in a middle in alignment with the keyway of said base for the passing of said key, and a plurality of inverted T-shaped control slots disposed at two opposite sides adjacent the elongated slot of said actuating plate, a thickness of said actuating plate being thinner than a depth of the front recess of said base;
• a plurality of retainer elements respectively mounted in said retainer element receiving chambers and said spring receiving chambers, each retainer element comprising a flat tail received in one retainer element receiving chamber, a flat guide plane perpendicularly extending from one end of said flat tail to guide the movement of the respective retainer element in a fixed course, an arched front contact tip extending forwards from said flat guide plane and partially projecting into the keyway of said base, and a lug at one side of said flat guide plane inserted into one control slot of said actuating plate;
• a plurality of second spring elements respectively mounted within said spring receiving chambers and stopped against the flat tails of said retainer elements to force said retainer elements toward the keyway of said base;
• and
• a cover covered on said base to hold said actuating plate inside the front recess of said base, said cover comprising an elongated slot in a middle in alignment with the elongated slot of said actuating plate and the keyway of said base for the insertion of said key, two backward hooks respectively forced into the side notches of said base and hooked in the retaining holes of said base.