Calibre 12-600 AT was introduced by Patek Philippe in 1953 and became a classic, loved by watch enthusiasts today. Its successor, 1960's Cal. 27-460, was an evolution of that famous movement and lasted in production until 1985, though the company had introduced many other movements in the intervening 25 years. Walt Odets called Cal. 27-460 “the reigning queen of that half century” and “the most elaborate and costly simple automatic watch movement ever produced.”
The name of Cal. 27-460 explains its key features
- The movement was 27 mm (12 ligne) in diameter
- The movement was 4.60 mm thick
Like Cal. 12-600 AT, this movement features a solid 18 karat yellow gold rotor, consummately decorated with engine turning even though no watch at the time had an exhibition case back. Timing was kept using Patek Philippe's patented Gyromax balance wheel, and this wheel beats at 19,800 A/h. Aiding the Gyromax wheel is an adjustable balance spring stud carier, replacing the swan neck regulator in the predecessor, and self-compensating Breguet balance spring. The other main change from Cal. 12-600 AT is a revamped rotor ball bearing, a source of trouble due to the massive rotor.
The design calls to mind Patek Philippe's mid-century hand-winding movements. Separate cocks are used for the balance wheel, escape wheel, and fourth wheel. A large bridge is used for the third wheel and center wheel and also acts as a mounting point for the rotor bearing. The movement is rhodium plated and finished to the standards of the Geneva Seal. Cal. 27-460 lasted in production for 25 years, powering most of Patek Philippe's high-end watches and complications. It was finally retired in 1985, a remarkably long run for a 1950's design, and replaced by the micro-rotor Cal. 240.