When Bvlgari unveiled the Octo Finissimo Ultra in 2022, it was the world’s thinnest watch at just 1.8mm thick, dethroning the 2mm Piaget Altiplano Ultimate Concept, which had held the record for the world’s thinnest mechanical watch since 2018. But in that same year, Richard Mille came out of leftfield with the RM UP-01 Ferrari, nabbing the title by a very slight margin of 0.05mm. But as it turns out, Bvlgari was not done. It made a return to the throne this year with the Octo Finissimo Ultra COSC, clocking in at an unbelievable height of 1.7mm.

But as is often the case, focusing on numbers alone runs the risk of obscuring much of what it really takes to create a watch this thin. The Ultra COSC achieved the new record by further reducing the thickness of the crystal and the bezel of the original by 0.1mm. As its name suggests, the watch is a certified chronometer, making it not only the world’s thinnest mechanical watch but also the world’s thinnest chronometer.

The watch was conceived in partnership with complications specialist Concepto. According to Valérien Jaquet, founder and CEO of Concepto, it was the result of three years of development.

“The brief was very simple: invent the world’s thinnest watch with chronometric criteria, without having to use any tools for winding and setting the time.”

Valérien Jaquet, founder and CEO of Concepto

The architecture of the Ultra COSC

The Ultra COSC relies on a set of best practices in the history of ultra-thin watchmaking. This includes doing away with a base plate by having its case back milled to accommodate the movement parts. The concept of reducing the basic architecture of the movement began in 1976 with the revolutionary but famously unreliable Jean Lasalle calibers 1200 and 2000. The majority of the wheels including the crown wheel were inset into the base plate and supported only on one side. There were no bridges except for balance wheel bridge. Instead, each wheel ran on ball bearings with the races installed in the base plate. This eventually opened doors to the idea of eliminating the base plate completely and using the case back to support the moving parts.

The first mechanical watch to adopt this approach was the Audemars Piguet Ultra-Thin Automatic Tourbillon ref. 25643 in 1986. At launch, it was the thinnest tourbillon watch ever produced, with a height of 4.8mm. The movement was created by André Beyner and Maurice Grimm, the latter of whom had previously developed the impressive Dinosaure ultra-thin quartz watch for Omega six years earlier. The Dinosaure was only 1.42mm thick in total, with the caseback serving to support the driving gears for the hands.

The case of the Ultra COSC measures 40mm wide and consists of four components to be precise — a caseband, onto which the case back is inset and attached, a gasket and a bezel. Thinning the watch to 1.7mm from its previous height of 1.8mm involves shaving off 0.1mm off its crystal as well as the bezel. This brings the clearance between the hands and crystal down to just 0.09mm. As a safety measure, the bezel is cushioned by four silicone bumpers, which prevent the crystal from making contact with the hands under pressure. While the bracelet, bezel and case band of the Ultra are made of titanium, the case back accommodating the moving parts is made of tungsten carbide, which is more rigid than titanium.

Another technique it took from the playbook is arranging all gears horizontally on a single plane, meaning the motion works, second wheel and fourth wheel are spread out. To put things into perspective, a conventional central-seconds watch has its fourth and second wheels located on the same axis, with the cannon pinion and motion works sitting above the latter, carrying the hands one on top of the other. But to reduce thickness, all these wheels have to be spread out on the horizontal plane, with the barrel driving the motion works via an auxiliary train.

Notably, the Ultra COSC goes a step further in deconstructing the motion works. Specifically, it separates the hour and minute wheels from the cannon pinion. In a conventional movement, the cannon pinion to which the minute hand is attached, is friction fitted to the centre pinion shank. It drives the minute wheel that carries the minute pinion which in turn drives the hour wheel mounted on the cannon pinion. In the Ultra COSC, an additional wheel is introduced right at the start, driven directly by the barrel. This wheel drives an intermediate wheel that in turn drives the minute wheel, which then powers the hour wheel pinion, displaying the hours. Simultaneously, the intermediate wheel drives a transmission wheel below it, which drives the cannon pinion for the minute display. This intermediate wheel is part of a differential, which will be explained shortly.

The fifth wheel in the wheel train is a solid wheel that serves as a seconds indicator. However, due to the additional wheel in the going train, the fifth wheel is rotating counterclockwise. As such, the seconds chapter ring is directly printed on the fifth wheel and requires a separate pointer printed on the bridge below so the seconds can be read more intuitively.

The going train is supported by a skeletonized bridge but the barrel has a so-called “hanging” construction, which was invented by Lépine. A hanging barrel is only supported at its bottom pivot, which helps to reduce the overall height of the movement. Another thing that’s unusual about this barrel is that the ratchet wheel doubles as the lid to further reduce thickness. The barrel arbour has four protrusions onto which the ratchet wheel is press-fit to create a tight seal and secured with a screw. It lies on the same plane as the hands, going train bridge and balance wheel.

The new hand-winding and time-setting system

Another aspect of the Octo Finissimo Ultra COSC that represents a radical departure is the elimination of the keyless works entirely. Typically, a watch’s keyless works include components like the winding stem, winding pinion, clutch, and various levers that allow the user to wind the mainspring and set the time through the crown. These parts take up both vertical and horizontal space. Typically, the crown wheel (responsible for winding the mainspring) is situated on the bridge side of the movement, while the motion works (responsible for driving the hands) are located on the dial side. The keyless works enable these two functions to be controlled by the crown, which can be pulled out to different positions to engage either train.

However, in the Ultra COSC, these traditional components are absent. Instead, the winding and setting trains are arranged on the same plane on either side of the watch. This allows them to be directly engaged by two flat “crowns” on the edge of the case back, which can be operated by simply using your fingertips. It is the only one of the three most recent record holders that can be wound and set without specialized tools.

The planar crown at 8 o’clock directly winds the barrel via a crown wheel with an integrated ratchet and winding clicks, which ensures that the crown wheel effectively transmits rotational force to the mainspring barrel while preventing any reverse movement. On the other side, the 3 o’clock planar crown directly drives an intermediate setting wheel which likewise has an integrated ratchet mechanism to prevent any reverse motion. These solutions effectively eliminate the need for a clutch (castle wheel) and a winding pinion which are perpendicular gears that would take up the most height in the keyless works.

Due to the complete reconfiguration of the second wheel (traditionally the center wheel), cannon pinion, and hour wheel, a new solution is required to allow precise setting of the hands without impacting the gear train. In a traditional movement, the cannon pinion functions as a friction clutch. It is friction-fitted onto the center pinion shank and rotates with it while the watch runs. However, during hand setting, the cannon pinion can slip on the center pinion shank thanks to an indenting system. In this altered arrangement of the center wheel, cannon pinion, and hour wheel, this traditional setup is no longer viable. Instead, a differential is employed to enable time-setting without interfering with the operation of the gear train.

There are two wheels visible on the dial that forms part of the differential. These are input wheels; the uppermost wheel receives continuous motion from the gear train and the middle wheel is driven by the crown during time-setting. However, there’s an output wheel beneath that drives the motion works. The output wheel combines the inputs of the uppermost wheel and the middle wheel, meaning the speed at which it rotates is a weighted combination of the speeds of the input wheels. This allows independent time-setting adjustments without affecting the uppermost wheel and the gear train.

Redesigning the escapement and balance assembly

The balance wheel is free-sprung with gold turnable weights that are recessed into its rim and is attached to a flat spring. Normally, the outer end of the hairspring is pinned to a stud that is attached to a mobile stud holder by a screw which is clamped to the balance cock. Here the stud holder is integrated into one of the arms of the three-armed upper balance bridge. Notably, the bridge is also designed with a degree of flexibility to act as a shock-absorption system for the balance, enabling it to do away with the usual setup of upper and lower jewels, chaton and springs, significantly reducing height.

The upper balance bridge also supports the escape wheel, and the entire balance bridge is screwed to a circular lower bridge, similarly designed as a shock-absorption system. This modular construction enables entire balance assembly unit to be conveniently removed during servicing. The pallet fork is supported by a small bridge that is attached to the lower bridge and the banking pins are embedded into the lower bridge.

As the entire oscillator is housed in a shock-absorbing unit, the escapement was able to do away with a safety roller and guard pin. In a conventional movement, a safety roller (a small disc with a semicircular cutout attached to the balance staff) and a guard pin (a thin raised pin attached between the fork horns) work together to prevent overbanking, which occurs when the watch is subjected to a sudden jolt and the roller jewel swings too far and ends up behind the horn of the pallet fork, causing the watch to stop. They take up the most height in an escapement assembly and to eliminate them, the pallet fork had to be redesigned.

Obtaining COSC Certification

Achieving chronometer status is a significant accomplishment as there is an inherent tension between achieving extreme thinness and maintaining chronometric performance, which is the ability to keep accurate and consistent time. Chronometric performance is dependent on numerous factors including balance inertia, frequency, consistency of torque over power reserve, escapement design, isochronism, finishing, positional adjustments, regulation and assembly.

Ultra-thin movements sacrifice chronometric potential for the sake of slimness. They tend to suffer from weak balance power as horizontal space is spent accommodating gears on a single plane. As a result, there is limited space for both the balance wheel and mainspring barrel. A high frequency/ inertia balance requires a longer or thicker mainspring while a balance with a higher moment of inertia requires more room. Moreover, the lack of vertical space usually precludes the use of an overcoil.

The challenge is to extract maximum performance within these constraints. As Valérien explains, “Our prototyping watchmakers have conducted extensive practical tests, allowing us to identify the optimal thickness and cross-sections needed to achieve the desired functions.”

The Ultra COSC benefits from its wide octagonal shape. While the general design appears to be a circle within an octagon, the movement parts are not confined within a circle. Valérien points out that the bezel is cut to accommodate protrusions of the barrel as well as sub-dials. The mainspring barrel is large as a result, enabling it to house a long mainspring for a power reserve of 50 hours. Additionally, the mainspring has a height of 0.83 mm, which is almost 50 percent of the watch’s total height, allowing it to deliver substantial torque for a watch this thin. As such, the balance is able to have an inertia of 8mg.cm2 and a frequency of 4Hz, which impressively, is comparable to an ETA 2892.

Apart from being the more streamlined option, the use of a free sprung balance also plays a crucial role as it offers better isochronism, maintaining accuracy regardless of how much it swings. The aspects, ultimately, that have a significant impact on performance is the extreme care taken during assembly, adjustment and regulation.

Valérien elaborates, “The hairspring and balance wheel must be meticulously sorted and then paired together to achieve the perfect results. For the movement to work, the balance wheel and pallet fork need to have micron-precise radial play, a much tighter tolerance than for a normal movement, and far beyond what is achievable with production tolerances. To achieve the correct radial play, we must measure and sort all the corresponding jewels in-house to match them with the correct diameter of the pivots.”

“Our most skilled watchmakers spend hours and hours achieving these micron-precise endshakes and correctly regulating the movement,” says Valérien. Naturally, as a result, the COSC Ultra will never be a product of a high-volume output; it must be assembled, adjusted and regulated by a single, highly skilled watchmaker.

Given that the mainplate is also the case back and it lacks a physical seconds hand, you might wonder how the movement was tested at the COSC. Valérien explains, “For small series, the COSC takes measurements with the help of a physical person, rather than an automated machine. There’s no need for test dials or hands. On the other hand, we had to create a specific movement holder to enable the COSC to carry out its measurements in the different positions and to wind the movement.”

Being a chronometer only adds to the list of features that has set the Ultra apart from previous record holders. This includes being the only one with conventional hands, a seconds indicator, tool-free time-setting and winding, and, perhaps equally impressive, how it fits into the broader, inimitable design language of the Octo Finissimo.

Tech Specs: Bvlgari Octo Finissimo Ultra COSC

Movement: Manual-winding COSC-certified caliber BVL 180; 50-hour power reserve
Functions: Hours and minutes; small seconds
Case: 40mm; sandblasted titanium; no water resistance
Dial: Openworked with PVD-treated counters
Strap: Titanium bracelet with integrated folding clasp
Price: USD 529,000
Availability: Limited edition of 20 pieces