Worlds Thinnest Automatic Tourbillon for 32 years- Audemars Piguet Caliber 2870 in the Ref. 25643

The Audemars Piguet Caliber 2870, as featured in the world’s thinnest tourbillon wristwatch of its time, is more than just a watch movement. It is a symbol of the brand’s legacy, a testament to the art of watchmaking, and a beacon of innovation and craftsmanship. Its creation not only marked a significant moment in Audemars Piguet’s history but also left an indelible mark on the entire horological world.

Introduced in 1986, the Caliber 2870 showcased Audemars Piguet’s relentless pursuit of combining technical prowess with aesthetic elegance. The movement, measuring a mere 4.8mm in thickness, broke new ground in the world of luxury timepieces, redefining the boundaries of ultra-thin watchmaking. Audemars Piguet achieved this remarkable thinness through several innovative design choices. The movement’s mainplate doubles as the baseplate for the tourbillon, eliminating the need for a separate bridge. Additionally, the use of a hammer rotor, as opposed to a more traditional central rotor, contributes to the movement’s slim profile while allowing an unobstructed view of the tourbillon through the watch’s sapphire crystal.

The tourbillon, a complex mechanism designed to counteract the effects of gravity on a watch’s accuracy, is a testament to the craftsmanship and technical skill of Audemars Piguet’s watchmakers. Integrating this intricate feature into such a slim profile was a formidable challenge. Remember, this is long before things like CAD and spark erosion machines and LIGA technology made tourbillons within reach for a wide range of makers, so this was an almost impossible challenge. The Caliber 2870’s tourbillon cage, one of the smallest & lightest ever made was 7.2 mm in diameter, and about 2.5mm high, and it is made of titanium – the first use ever recorded of such a material in tourbillon manufacturing and it weighed in at an astonishingly light 0.134 grams, a feat that further underscores the technical brilliance behind its construction. The Caliber 2870 was not just a technical marvel; it was also a design masterpiece. The choice of materials played a crucial role in achieving the desired aesthetics and functionality. Titanium significantly reduced the weight of the tourbillion cage and enhanced the efficiency of the movement.

The introduction of the Caliber 2870 had a lasting impact on the watchmaking industry. It pushed the boundaries of what was thought possible in mechanical watch design and inspired a wave of innovation in ultra-thin timepieces. This groundbreaking movement set a benchmark for future generations of watchmakers, demonstrating that technical limitations could be overcome with creativity and precision engineering.

The automatic winding system is also very unusual. The design of the movement, as well as the goal of keeping the watch as flat as possible, precluded the use of a full movement diameter rotor, and instead, caliber 2870 uses a “hammer” winding system, in which a platinum-iridium weight swings through a small arc rather than a full circle – the hammer is visible in the aperture at 6:00 on the dial. There is no winding crown – instead, you gently swing the watch to charge the mainspring – and hand-setting is via a small, flat crown set into the case back.

The Audemars Piguet Caliber 2870 stands out primarily due to its innovative gear train design, which sets it apart from traditional watch movements. Typically, a watch movement includes a “bottom plate,” facing towards the dial. This plate is termed as such because when a watch is disassembled by a watchmaker, it’s usually placed face down on the bench, making this part of the movement the lowermost layer. In contrast, the “top plate,” as viewed from a watchmaker’s standpoint, could either refer to the bridges in a bridge-type movement or a literal top plate in designs with a three-quarter or full plate.

Ordinarily, the gear train’s pivots are embedded in jewels situated in the bottom plate on one side, and in the top plate or bridges on the opposite side, with the entire assembly housed within the watch casing. In a striking departure from this norm, the Caliber 2870 by Audemars Piguet dispenses with the use of traditional bridges or a standard top plate. Remarkably, the pivots for both the gear train and the automatic winding mechanism are set into jewels that are incorporated into the watch’s case back, serving as the movement’s top plate.

This approach is rare, with only a few other watches employing a similar principle, such as the Piaget Caliber 900P. This caliber introduced further innovations, including aligning the hands and dial with the going train.

The unique construction of the Caliber 2870 is a significant reason why it’s often identified solely by its caliber number rather than a model reference. In this design, the movement isn’t merely a component; it essentially constitutes the entire watch.

At the heart of the dial’s design lies the prominently featured tourbillon, which completes a counterclockwise rotation every 50 seconds. This tourbillon is artistically presented as a sun, complete with slanted “rays” spreading across the dial. Audemars Piguet drew inspiration for this motif from an ancient Egyptian relief depicting Pharaoh Akhenaten and his wife, Nefertiti, who are widely recognized in the West as the parents of Pharaoh Tutankhamun. The latter, commonly known as King Tut, gained significant attention in popular culture around 1986, largely due to the immense success of “The Treasures of Tutankhamun,” a major museum exhibition that concluded its tour in 1981. This exhibition was one of the first and most significant blockbuster museum events, making King Tut a prominent figure during that period.

Historically, the creation of very small tourbillons was not unprecedented. In 1927, James Pellaton crafted a tourbillon with a mere 23.6 mm diameter, and his protégé, Fritz-Andre Robert-Charrue from Le Locle, who later became the director of the watchmaking school there, dedicated five years to developing an exceptionally tiny tourbillon movement of just 19.7 mm in diameter, featuring an 8 mm carriage. Remarkably, this achievement in terms of diameter still stands unmatched. Robert-Charrue completed a watch housing this movement in 1945, measuring just 23.8 mm in total.

Although technically feasible, producing tourbillons small enough for wristwatches on a larger scale was a significant challenge, explaining the rarity of wristwatch tourbillons intended for public sale prior to the introduction of the 2870. Patek Philippe did create some wristwatch tourbillon movements in the 1940s and 50s, but these were limited to VIP clientele or for observatory timing competitions. Omega’s caliber 30I tourbillon movement, introduced in 1947, followed a similar path, aiming for precision and stability over aesthetics, and was not a commercial product. Only 12 of these were made, and one notably won first place at the Geneva observatory competition in 1950. This caliber, featuring a 7.5-minute carriage, along with Patek’s limited observatory tourbillons, formed the early generation of wristwatch tourbillons.

Other unique examples include the French company Lip’s prototype tourbillon movements based on their T18 caliber and a wristwatch tourbillon created by Edouard Belin in 1930 using a Lip ebauche at the Besancon watchmaking school. In 1890, Girard-Perregaux even crafted a 30 mm/13 ligne tourbillon movement with a chronometer detent escapement. However, these instances were notable for their rarity and uniqueness, primarily serving as prestigious symbols for their creators rather than commercial ventures.

This background sets the stage to appreciate the groundbreaking nature of the Audemars Piguet Caliber 2870 as it was the first to offer a self-winding, series-produced tourbillon. Before its release in 1986, tourbillon wristwatches were exceedingly rare, virtually non-existent in series production. The Audemars Piguet Caliber 2870 wasn’t just a novelty; it represented a significant leap in watchmaking, paving the way for the diverse range of tourbillons available in the current market.

From 1986, ref. 25643 by Audemars Piguet was the first series-produced tourbillon, the first series-produced automatic tourbillon, and for decades, the world's flattest tourbillon. The Audemars Piguet Caliber 2870 was only 4.8mm thick and maintained its remarkable status as the thinnest automatic tourbillon ever produced all the way up to 2018 when Bulgari launched the peripheral rotor Octo Finissimo Tourbillon Automatic, which was 3.95mm thick. Not even 1mm thinner. Remarkable that the 2870 held its record for 32 years.

Beyond its technical achievements, the 2870 marked another major milestone as the first tourbillon wristwatch produced for a broader, albeit still exclusive, market, rather than being a one-off piece or designed solely for observatory chronometry competitions. It combined advanced technology with a strong focus on style and design, made it a trailblazer in modern tourbillon wristwatches. This innovation was as significant for Audemars Piguet in 1986 as the introduction of the Royal Oak was in 1972, underscoring the brand’s legacy in shaping contemporary watch design.

Hairspring.com said

“This watch would still be impressive if released today. It was a loss leading project that only was green lit to show the world that AP had technical ability to rival the best, much like the RD series today.”

#2870 #AP2870 #AP25643 #HauteHorlorgerie #Vintage #VintageAP

The most beautiful variant is this MOP with full gold bracelet.

It was for sale at Touch of Modern for only $27,325- and it could have been a decade ago. The only issue with ToM is you wouldn’t have known whether it was in working order and if it would’ve needed to be sent off for a $5000 service. I’m currently looking at acquiring one of these for the collection and that’s why I went down this rabbit hole. If you know of a MOP dial 25643 please DM me. Heck, if you know of ANY good/excellent condition examples, please contact me.

If you read this entire thread, I commend you. I thought it was all rather interesting so I decided to write up a detailed post explaining what I found. Enjoy!