Calibre 340

Notes

At the end of the 1950s Longines began an initiative to update their offerings which prioritized the development of thinner automatic movements designed with a direct central seconds and made with the intention of being able to easily accomodate a date function. The earlier 290 calibre family was the first incarnation of this new paradigm but was still substantially thick at 6mm. The 340 family of calibres, designed two years later, was the first of the new paradigm to accomplish the goal of a thin calibre that was automatic with central seconds and date complications. All these variants were the same thickness, 4.45mm which was a central feature of the design. Later iterations of this calibre that seem functionally identical to earlier ones (342, 343, 345, 352, 353, 355) are actually slight updates in both the functioning of the date mechanisms on the date models and in the machining methods on the main plate. Longines was focusing on getting better statistical control of their machining processes to better ensure interchangeability of parts between movements. These updates were experimentations in achieving finer tolerances in the manufacturing process.
340, 342: center second
341, 343, 345: center second, date
350, 352: sub second
351, 353, 355: sub second, date