Cosmograph Daytona Zenith Daytona 16520 — Mk 1, Floating Cosmograph

Rolex 4030

Self-windingAutomaticA mechanical movement wound by a rotor as the watch moves on the wrist. chronographChronographA watch function for starting, stopping and resetting an elapsed-time measurement independently of the ordinary time display. built from the Zenith El Primero 400. Rolex replaced or altered over half the parts (some 200 modifications), removed the date, fitted a larger free-sprung balanceFree-sprung balanceA balance regulated by changing its inertia with adjustable weights or screws, while the balance spring’s active length remains fixed. It dispenses with the conventional index regulator that changes the spring’s working length. The term describes the regulating arrangement, not a particular hairspring material or certification. and slowed it from 36,000 to 28,800 vph, then submitted it for certification. Used in the 16520 family until 2000.