While Patek Philippe had an excellent and respected automatic watch movement in their Cal. 315, the company set about improving it in the 2000's. The result was Cal. 324, an improvement in almost every way even though the basic architecture and shape was the same as its predecessor. The first notable difference was the oscillation rate: Cal. 324 beats at a now-standard 28,800 A/h for improved accuracy and consistency. The rotor is also larger, increasing power generation to reach a respectable but not excellent 35-45 hours of reserve of power.
Inside, Cal. 324 has a new-generation Gyromax balance wheel with four spokes (up from 2) and four tunable weights (down from 8) placed around the rim rather than inside it. This new movement also features new wheel tooth geometry, a bipartite minute train bridge, asymmetric date train, reinforced winding wheel, and lower-friction wheel arbor pivots.
The basic model, Cal. 324 S C, measures 27 mm in diameter and 3.3 mm thick. Patek offers a number of complications on this platform, increasing the thickness to as much as 5.78 mm, and also offers wider models at 30, 31, 32.6, and even 33.3 mm in diameter.
Cal. 324 is Patek Philippe's mainstream automatic movement and features a full-sized central winding rotor. The company also uses a micro rotor movement, Cal. 240, where a thinner model is required, and still equips a few models with the older Cal. 315 as well.
In 2006, Patek's Advanced Research project yielded its first Cal. 324 project: They paired the silicon escape wheel found on the Cal. 315 AR movement with a new "Spiromax" flat silicon hairspring. This was used in their Annual Calendar, Ref. 5350, this time with 300 pieces produced.
Patek pairs Cal. 324 with various complication modules:
- Cal. 324 S C - Center seconds, date (27x3.3 mm, 29 jewels)
- Cal. 324 S C FUS - Center seconds, date, second time zone (31x4.82 mm, 29 jewels)
- Cal. 324 S QA LU - Center seconds, annual calendar, moon phase (30x5.32 mm, 34 jewels)
- Cal. 324 S QA LU 24H/206 - Center seconds, annual calendar, moon phase, 24 hour time (32.6x5.78 mm, 34 jewels)
- Cal. 324 S QA LU 24H/303 - Center seconds, annual calendar, moon phase, 24 hour time (33.3x5.78 mm, 34 jewels)
- Cal. 324 S IRM QA LU - Center seconds, annual calendar, moon phase, power reserve indicator (30x5.32 mm, 36 jewels)
- Cal. 324 S QR - Center seconds, date, retrograde perpetual calendar (28x5.35 mm, 30 jewels)