Zenith Caliber 400 and 4021

Zenith Caliber 400 – The Foundation of El Primero

The Zenith Caliber 400 is the modern successor to the original El Primero movement introduced in 1969. It is known for being one of the first fully integrated automatic chronograph movements and remains one of the most respected mechanical chronograph calibers in production.

Key Specifications:

• Type: Automatic, integrated chronograph

• Diameter: 30.00 mm

• Thickness: Approximately 6.5 mm

• Jewels: 31

• Beat Rate: 36,000 vibrations per hour (5 Hz)

• Power Reserve: Around 50 hours

• Chronograph Features:

  • Central chronograph seconds

  • 30-minute counter (3 o’clock)

  • 12-hour counter (6 o’clock)

  • Small running seconds (9 o’clock)

• Additional Function: Date window at 4:30

• Chronograph Control: Column-wheel with horizontal clutch

• Rotor: Bi-directional, mounted on ball bearings

The Caliber 400 maintains the same compact, high-performance design as the original El Primero 3019 PHC. It remains in use across many Zenith Chronomaster models and continues to set a benchmark for precision chronographs.

Zenith Caliber 4021 – Enhanced with Display and Power Reserve

The Zenith Caliber 4021 is an evolved version of the 400, designed for modern open-heart and power-reserve display watches. While it retains the high-frequency chronograph core, it adds more visual and functional complexity.

Key Specifications:

• Type: Automatic chronograph with power-reserve indicator

• Diameter: Approximately 30.45 mm

• Thickness: Around 7.75 mm

• Jewels: 39

• Beat Rate: 36,000 vibrations per hour

• Power Reserve: 50 to 55 hours

• Display Features:

  • Power-reserve indicator

  • Open-heart design (visible escapement and balance)

• Chronograph Features:

  • Same as Caliber 400 (central seconds, 30-minute and 12-hour counters, small seconds)

• Finishing: Often skeletonized bridges and a partially open dial to showcase movement components

• Upgrades: Improved escapement architecture in some versions, including the use of advanced materials like silicon for longer service life and resistance to wear

Caliber 4021 is often found in the Chronomaster Open series and other contemporary Zenith watches that emphasize both mechanical performance and modern aesthetics.

Side-by-Side Summary

Conclusion

The Zenith Caliber 400 remains one of the most iconic and reliable high-frequency chronograph movements in watchmaking. The Caliber 4021 builds on that legacy with modern refinements, including aesthetic enhancements and functional upgrades such as the power-reserve display and skeletonized structure. Both movements represent Zenith’s technical mastery and enduring design philosophy. Whether you prefer the classic look of the 400 or the modern openness of the 4021, both deliver performance rooted in one of horology’s most important innovations.