Calibre 4057 B is an automatic central-foudroyante flyback chronograph movement from Zenith. Introduced in 2011, this was one of a series of modern movements expanding the classic El Primero movement family.
Although the El Primero operated at 36,000 A/h from the very start in 1969, no model allowed true 1/10 second chronograph precision. Zenith worked on various approaches to the problem and decided that a 1/10th foudroyante (one revolution per second) central seconds hand was the most straightforward. The outer scale on the dial is divided into 10 major sections, with each subdivided with 10 markers and a “0.5” in the center.
Since the existing 60 seconds markers on the dial to not line up exactly with this new 100-marker scale, a novel approach was adopted to ensure the central seconds hand always lines up properly Rather than use the existing chronograph brake, a new 100-tooth gear stops the seconds hand, ensuring it always points at one of the 100 marks along the outer track.
Another technological difference in Cal. 4057 is the power delivery. Rather than driving the chronograph from the seconds wheel as is typical, it is driven from the escapement wheel. This allows true 1/10 second precision. To avoid impacting isochronism, the clutch is a single lightweight piece of silicon that remains continuously engaged.
Cal. 4057 is a “B” El Primero movement, meaning that the date window is at 6 00 rather than the traditional and unconventional 4 30 placement of the classic El Primero movement. This version adds a flyback feature to the foudroyante El Primero introduced in Cal. 4052 the previous year.