Born in 1977 out of the need to create a mechanical movement that could rival quartz movements in thinness and wearability, the Caliber 240 has been the star of many collections within Patek Philippe’s offerings. Serving as a beautifully finished ultra-slim movement, the basic Caliber 240 can be found in many time-only references, such as the 5120, 7200, and we must not forget the iconic, yet underrated reference 5026. These references all have a theme in common despite their very different styles, and that is their dainty size. Easily coming in under 10mm in thickness from case back to front crystal, these watches exude elegance, class and a certain… if I may… timelessness.

The caliber 240 is one the few modern calibers that has undergone very little change in terms of usage of specific materials to produce it. There have been changes in the way that the oscillating weight is manufactured. Going from an invisible attachment style that hides the screws and fully shows the micro-rotor, to the now standard attachment style that shows you the two mirror-polished screws that hold the micro-rotor in place.

Next is the decision to discontinue the use of steel ball-bearings in the micro-rotor in favor of the zirconium ball-bearings – a huge improvement in the efficiency of the automatic winding.

The caliber 240 eventually benefitted from the use of Silinvar hairspring. This improvement added a higher level of accuracy, and reliability to this caliber. The use of Silinvar, the adoption of zirconium ball bearings for the automatic winding, and many other minor, yet invisible changes to this caliber helped elevate an already good movement to one truly great among greats. Today, this caliber boasts an amazing daily rate of -3/+2 seconds (oftentimes it can easily maintain -1/+1 in my personal experience working on them), and over 48 hours of power reserve.

If you are a seasoned collector, you may have noticed that some power reserve indicator dials like that of a ref. 5712 show “48” as the final marker, while others include an additional dot after 48. This subtle difference reflects the gradual increase in the movement’s power reserve over the years. Originally, the caliber 240 offered around 48 hours of reserve, but today, I have seen this movement reach right around 52 hours. The addition of this extra dot on the dial acts as a gauge for the reserve, subtly under-promising while over-delivering in terms of performance.

Outside of the observable changes or improvements, the caliber 240 remains the same after nearly 50 years in production.

The Base Caliber

The name of this caliber comes precisely from its thickness when you remove the micro-rotor. The movement measures exactly 2.40mm in thickness from the dial side to the bridge side (excluding the length of the center post upon which the cannon pinion and hour wheel turn). When you add the micro-rotor, it measures 2.53mm in thickness. Considering that this is a self-winding movement, that’s very thin for a movement of this kind.

Achieving a thickness of 2.53mm would not have been possible without fully simplifying the automatic winding mechanism – shifting from a standard central rotor to an off-center design with a significantly reduced diameter. This is perhaps the simplest automatic winding mechanism found in any movement in the world. Boasting an uninterrupted connection of gears from the micro-rotor itself all the way up to the sliding pinion on the dial-side, the movement could not be any simpler. Six components make up the entire automatic winding system.

Automatic winding mechanism

1. Micro-rotor (Removed in the photo)
2. Uncoupling wheel
3. Winding wheel
4. Click
5. Intermediate winding wheel
6. Rachet-driving wheel

The micro-rotor is manufactured out of solid 22K gold. It is mounted off center on at stainless steel wheel equipped with ball bearings. This design allows the rotor to easily swing in either direction with even the slightest movement of the wrist. The rotor is attached to the wheel using two mirror-polished steel screws. This assembly then drives the uncoupling wheel.

The uncoupling wheel, consisting of two ball bearing wheels joined by a steel platform, assumes the responsibility of what you would normally find here in a common self-winding movement: the reverser wheels. In a typical reverser wheel system, there are two reverser wheels that interact with each other and with the rotor. The reverser wheels convert the bi-directional turn of the rotor into uni-directional turns of the winding wheel. The reason for this is that the mainspring can only be wound in one direction, but your rotor swings in two directions. Reverser wheels solve this problem. However, Patek Philippe further simplifies the solution.

Patek Philippe opted for this double ball bearing system that drives the winding wheel when it turns in one direction, and moves away from the winding wheel (uncouples) when it turns in the opposite direction.

Born as an incredibly simple self-winding movement, the caliber 240 was conceived with the intention of serving as the base for the most highly regarded complications within the Patek Philippe arsenal.

The great wheel (bridge side/movement side) of this caliber also drives the intermediate setting wheels and minute wheel on the dial side, providing the buttery smooth friction that we feel when setting the time. This is thanks to its design, with a total of 4 components that make up this one wheel, it simultaneously interacts with the barrel, third wheel, 2nd intermediate setting wheel, and the minute wheel. Despite the micro-rotor being in the spotlight, the great wheel is the true star of the show!

The use of this complicated great wheel allows the manufacturer to shave considerable thickness off the caliber. Moving up the gear train layout, we have the barrel/rachet wheel, third wheel, fourth wheel, and escape wheel. This area of the movement is engineered and made the same as your average caliber, only thinner.

The decision to implement this complicated four-piece great wheel also frees up a lot of main plate real estate below the dial, allowing for a wide range of complications to be stacked on top of the base caliber. Simple calendar modules, perpetual calendar, World Time, and most impressively, the iconic celestial complication. For perspective, the base caliber has a total thickness of 2.53mm, while the addition of the World Time complication brings it to 3.88mm – an increase of just 1.35mm.

Complications

Some of the most well-known complications produced by Patek Philippe are based on the caliber 240. Often when I speak with collectors about Patek Philippe I can hardly talk about the brand without mentioning the World Time ref. 5110. It is loved by most, and rarely seen in action.

Coming in at a total of 239 parts and measuring only 3.88mm, the caliber 240 HU World Time is a relatively simple mechanism considering that the base caliber consists of 161 parts.

Twenty-four cities, twenty-four time zones, day and night indicator disk, twelve-hour display, twenty-four-hour display sharing a home with the day and night indicator disk, all coming together to work at the command of an articulated arm which then connects to a single pusher in the case. That is quite impressive.

Sitting on top of your base caliber is the double hour wheel. The lower hour wheel interacts with the minute wheel to keep track of time. The upper hour wheel, which has a twelve-point star, is joined to the lower hour wheel via a “C” spring and a steel “finger”. These components, under the tension of the C-ring, finger, and twelve-point star stick together to make up the double hour wheel. This double hour wheel allows you to keep track of time during normal operation of the watch, and allows you to jump to the next hour or time zone when pressing the pusher. When the pusher is pressed, the upper hour wheel “clicks” over one point of the twelve-point star, while the lower hour wheel remains in place. The upper hour wheel holds the hour hand. Every time the upper hour wheel moves one click, so does the hour hand and that’s the action that you see on the dial.

What’s Next

While we don’t know what’s next for the caliber 240, its proven efficiency in real-world use speaks for itself. Patek Philippe has certainly mastered the art of creating movements that endure in production for decades, evolving through subtle yet effective improvements over time. Gradual enhancements like the introduction of zirconium ball bearings, silicon components, optimized tooth profiles, and polished gear teeth exemplify the manufacture’s dedication to ensuring the longevity of the caliber 240, requiring only occasional fine-tuning as technology advances. This approach is supported by the company’s vast base of watches sold and serviced, providing invaluable real-world data. This practical experience, combined with a deep-rooted culture of planning for the future, ensures that its movements are engineered with a long-term vision, balancing tradition with modern advancements to secure its legacy for generations to come. This is all to say that there might be more innovations yet to come.

Final note

In conclusion, the caliber 240 is a remarkable feat of horological engineering that has stood the test of time, evolving with subtle yet significant enhancements over nearly five decades. Its ultra-slim design, combined with its innovative micro-rotor, has allowed Patek Philippe to offer some of the most elegant and functional timepieces. The movement’s simplicity and precision make it the ideal base for complex complications such as the World Time and celestial models, showcasing the brand’s dedication to technical mastery without compromising on wearability or aesthetic appeal. The caliber 240 remains a true icon within the world of fine watchmaking, a testament to timeless craftsmanship and innovation.