Audemars Piguet's "2120 Series": A Legendary Automatic Movement in Watchmaking History - A History of Masterpiece Movements

"Biographies of Masterpiece Movements" is arbitrarily selected and explained by Masamasa Hirota, editor-in-chief of Chronos Japan.
The first commemorative movement is the "2120 series" movement used by Audemars Piguet in the Royal Oak and other watches. Based on the 1967 Jaeger-LeCoultre Caliber 920, this movement has made its mark in history with its high-quality automatic winding, high aesthetic quality, and incredibly thin design.

Why the Audemars Piguet Caliber 2120/2121 is a masterpiece

　There are many excellent automatic movements in the world. However, there are not many that achieve a high level of aesthetics and practicality (or, more accurately, attempt to achieve both). In my opinion, the top of the list are Patek Philippe's Caliber 27-460AM, Chopard's Caliber LUC 1.96, and the Audemars Piguet 2120 series introduced here. All of these movements offer performance suitable for everyday use and have construction and finishes that will please connoisseurs.

　In the 1960s, various manufacturers began working on making automatic watches thinner. The biggest change was in the design. Previously, automatic watches from various manufacturers had been designed by layering a module on top of a hand-wound movement, but integrated automatic winding mechanisms, with the automatic winding mechanism built into the movement, became the norm. One such design was the micro-rotor, which offsets the rotor that winds the mainspring. Automatic watches from Universal Genève, Hamilton Buren, and Piaget, among others, succeeded in making their watches extremely thin by shifting the rotor and placing it in the same layer as the gear train.

　Meanwhile, Audemars Piguet's 2120 series, which pushed the limits of thinness by thinning the components, was based on Jaeger-LeCoultre's Caliber 920 ébauche (designed in 1967). The company supplied this excellent ébauche to Vacheron Constantin and Audemars Piguet, and later to Patek Philippe.

Compact switching rocker makes it possible to achieve a slim design

　The uniqueness of Caliber 2120 lies in its design, which places the second wheel, which drives the minute hand, at the center of the movement. If the second wheel were in the center, it would overlap with the center of the rotor, increasing the thickness of the movement. For this reason, micro-rotor automatic watches purposely offset the second wheel and rotor. However, if the second wheel is offset, the hands are more likely to jump when setting them. This is said to be the reason why manufacturers such as Audemars Piguet, Vacheron Constantin, and Patek Philippe were cautious about making their movements thinner.

　In contrast, Jaeger-LeCoultre's Cal. 920 uses a layout with the second wheel and rotor axle in the center of the movement. Despite this, the thickness is only 2.45mm for the Cal. 2120 without a calendar and 3.05mm for the Cal. 2121 with a calendar. This was made possible by the compact switching rocker automatic winding and the extremely small rotor axle.

　Although they are rare these days, the compact and wear-resistant switching rocker automatic winding mechanism was once essential for high-end automatic winding watches. However, due to its low winding efficiency, the switching rocker automatic winding mechanism had no choice but to be combined with a rotor made of a heavy material. This is why manufacturers who adopted the switching rocker automatic winding mechanism were almost exclusively those who could afford to spend money on the rotor.

Downsized rotor shaft

　The small rotor stem was also a great idea. Normally, making the rotor stem smaller increases the load on the stem. In contrast, the 2120 series has a round rail attached to the rotor, which slides on ruby ​​bearings embedded in the outer periphery of the base plate. This is a unique design that supports the rotor not only at the stem but also on the outer periphery.

It has been produced almost unchanged since 1967.

　However, the productivity of the extremely thin 2120 series was by no means high. At its peak, Audemars Piguet is said to have produced around 500 2120 series watches per month, but this was only possible because Audemars Piguet had skilled craftsmen.

　The Caliber 2120 has remained essentially unchanged since 1967, although it has undergone several minor changes. One is that the regulator was replaced with a free-sprung balance, replacing the regulator needle. The escapement was also changed from one made by Jaeger-LeCoultre to one made by another company (details have not been made public). Furthermore, in conjunction with the change to the in-house automatic Caliber 3120, the switching rocker of the Caliber 2120 was also redesigned to be more resistant to wear.

　Some say it's unsophisticated to expect aesthetic appeal from an automatic movement. Personally, I agree. Furthermore, the performance of the Cal. 2120 is far inferior to that of today's automatic movements. Its power reserve is only about 40 hours, and its winding efficiency isn't high enough to withstand desk work. Nevertheless, this movement, with its classic design and unique design, will likely remain a masterpiece among masterpieces for years to come.