5 minutes

The Magic Of Ultra-Slim Automatic Watches

By Jorg Weppelink

If you buy a new mechanical watch these days, chances are it’s going to be an automatic watch. If we go back to the 1950s and 1960s, the choice between a manual-winding or automatic timepiece was a very relevant one. That all changed with the quartz revolution in the 1970s. From that moment on, the question was no longer focused on your watch being manual or automatic, but instead on whether you wanted a quartz or mechanical watch, if you even asked at all. As we know, the quartz revolution kicked off a massive change in the watch industry to the detriment of mechanial timepieces, both manual-winding and automatic alike.

But did you know that even during this precarious era of mechanical watchmaking, some brands still achieved impressive mechanical successes? One of them was a result of the constant quest for creating thinner automatic watches. If you are wondering how automatic watches work, check out Tim Breining’s great article “What is an automatic watch and how does it work?”, where he explains the basic principle of automatic watches. In essence, automatic watches are wound through the motion of the wearer’s arm. As the wearer goes about their day, the rotor swings with every movement of their arm, winding the mainspring.

The rotor and the movement of the wrist is what keeps an automatic watch running.
The rotor and the motion of the arm are what keep an automatic watch running.

The rotor is, of course, an important part of any automatic movement. It’s the element that needs space to swing around when the wearer moves his or her arm. Luxury watches are equipped with rotors made from materials like tungsten, gold, or platinum to ensure they are heavy enough to swing into motion and wind the watch. The rotor, or oscillating weight as it’s also called, is thus crucial in creating thinner watches. Put another way, it’s a lot easier to create thin manual-winding watches because they don’t have a rotor, and as a result, don’t require the space for it in their cases.

Who developed the thinnest automatic watch movement?

However, this hasn’t stopped brands from trying to make automatic watches as thin as possible. Where there is a mechanical challenge, watch brands will try to find a way to solve it. There are some brands that have made a name for themselves over the years when it comes to manufacturing ultra-thin watches. Historically, Jaeger-LeCoultre, Vacheron Constantin, and Audemars Piguet competed to develop ultra-thin watches in the early 20th century. In the mid-1950s, Piaget also became part of this group. However, once the quartz revolution started, the quest for the thinnest watch moved to these timepieces. In 1979, Swiss manufacturer Concord launched the Concord Delirium, which measured just 1.98 mm thin. That movement was a major inspiration for the groundbreaking concept of Swatch watches that were introduced in 1983; the design used the case back as the mainplate to mount the movement on.

During the quartz reign, a number of brands put efforts toward developing a great ultra-thin mechanical movement. Jaeger-LeCoultre developed the now-legendary JLC caliber 920 that was introduced in 1967 for Audemars Piguet, Vacheron Constantin, and Patek Philippe. The company delivered the movement as an ébauche to the three brands. Essentially, that means it was delivered as a kit, and could be adjusted and decorated according to the wishes of the brand purchasing it – that’s where things got interesting.

The Audemars Piguet Royal Oak 5402 featuring the caliber 2121, aka JLC’s caliber 920.

The JLC caliber 920 was the thinnest automatic movement with a central-mounted rotor in the world at the time, but Jaeger-LeCoultre never actually used it for one of its own watches. Audemars Piguet used it as a base movement for the caliber 2120 that powered the first Royal Oak ref. 5402 introduced in 1972. Patek Philippe used the JLC movement to develop the caliber 28-255 for the first Nautilus from 1976, and Vacheron Constantin used it to develop the caliber 1120 that powered the legendary 222 from 1977.

Groundbreaking: Jaeger-LeCoultre’s caliber 920

Yes, this revolutionary movement powered three of the most iconic modern sports watches that would go on to start the revolution in watch design first sparked by Gérald Genta’s Royal Oak. An ultra-slim, reliable automatic movement was necessary to power these luxury watches and make Genta’s vision of a very slim automatic modern sports watch come to life. That’s exactly why the caliber 920 (or 2120 in the Audemars Piguet universe) was the perfect answer. The movement measured just 2.45 mm in thickness, and used a full-sized gold rotor. Besides being a technical marvel, the movement is also beautiful to see in action. The revolutionary architecture is gorgeous, as is the finishing from each of the three brands, even though display case backs weren’t common in those days. It was a joy to behold this mechanical marvel at work in the watches that did have one; seeing the full-sized rotor on the beryllium rail is simply wonderful.

For years, the JLC caliber 920 was the thinnest automatic movement with a full rotor, but how do you make something even thinner? The answer was – you guessed it – right there in the rotor. A watch rotor typically has to be mounted on top of the base movement, adding to the watch’s overall height, but there are other options such as integrating the rotor into the construction of the movement. This is where a micro-rotor comes into play. The micro-rotor was actually invented in the 1950s. This was the solution Piaget used for its groundbreaking 12P movement from 1960, which measured just 2.3 mm in height, 0.15 mm thinner than the caliber 920. That might not seem like much, but in watchmaking it certainly is. By the way, Piaget still uses micro-rotors in its ultra-thin calibers to this day.

Bvlgari and the Octo Finissimo Line

The same technique was employed by Bvlgari for its series of record-breaking Octo Finissimo watches. That series started in 2014 with the thinnest manual-winding tourbillon movement ever made in the Octo Finissimo Tourbillon, and the thinnest manual-winding minute repeater movement in the Octo Finissimo Minute Repeater. In 2017, however, we witnessed the introduction of the Bvlgari Octo Finissimo Automatic with a record-breaking new movement. The watch measures just 5.15 mm thick, and the caliber within, the BLV 138 with a platinum micro-rotor, just 2.23 mm. That same year, however, Piaget broke the record for the thinnest automatic watch with the 4.30-mm thick Piaget Altiplano Ultimate Automatic.

Bvlgari Octo Finissimo Tourbillon 102719 2
Bvlgari Octo Finissimo Tourbillon 102719

But Bvlgari took back the crown only a year later with the introduction of the Octo Finissimo Tourbillon Automatic in 2018. That watch became the thinnest automatic watch ever made, as well as the thinnest tourbillon watch. Unlike the previous record-breaking watches in the series, it did not use a micro-rotor. Instead, it used a so-called peripheral rotor, i.e., a very slim ring that rotates along the periphery of the movement. Essentially, you can integrate the rotor on the outside of the movement, making even slimmer calibers possible. While the first attempts at peripheral rotors date back to the 1950s, it was the brand Carl F. Bucherer that introduced the modern version in 2009 with its CFB A1000 caliber. Ever since, the peripheral rotor has inspired brands like Bvlgari, Vacheron Constantin, Breguet, and Audemars Piguet to come up with ultra-slim movements featuring this technical solution.

From the central rotor and the micro-rotor to the peripheral rotor, it’s all proof that the development of watches never stops. The quest to create ultra-thin watches is an ongoing one. Just last year, Bvlgari unveiled the manual-winding Octo Finissimo Ultra as the thinnest mechanical watch ever. That was quickly followed by the manual-winding Richard Mille RM UP-01 that currently holds the title. My guess is that we will see even thinner automatic watches in years to come, because in the end, it’s not just about breaking records, it’s also about showing how magical the world of mechanical watches truly is.

About the Author

Jorg Weppelink

Hi, I'm Jorg, and I've been writing articles for Chrono24 since 2016. However, my relationship with Chrono24 goes back a bit longer, as my love for watches began …

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