by Walt Odets

Side viewCase detailA diminutive 5.4 millimeters across its thickest section, the Jaeger LeCoultre Ultra Thin looks no more than two millimeters on the wrist. Perhaps that low profile is one reason it is so often ignored by wristwatch enthusiasts. Perhaps it is the lack of chronograph, second time zone, moonphase, day of the week, date, or seconds. Or even a minute track. Perhaps it is a dismissal of the Ultra Thin as merely a "dress" watch. Regardless, with its slim, elegant Dauphine hour and minute hands and five-minute dagger markers, the Ultra Thin is an almost flawless piece of design and engineering. In person, this is a watch with a shimmering, ethereal perfection that cannot be conveyed in photographs. It is a masterpiece. Perhaps more than any other current-production Jaeger LeCoultre, the Ultra Thin stands as testimony to the extraordinarily broad talents and deep quality of a manufacturer that, taken on the whole, is arguably the great house, among the greats, of contemporary Swiss watch manufacturing.


Disassembly trayThe Ultra Thin is a sophisticated product of Jaeger LeCoultre excellence in the guise of a very simple watch. To truly understand the Ultra Thin requires looking at it in detail, literally piece by piece.

Originally manufactured with the caliber 839, a movement dating from 1975, later production switched to the caliber 849. The two movements, nearly identical but for a few details, are 21,600 beats-per-hour, handwind calibers of approximately 9 lignes (20.2 millimeters) and 1.85 millimeter thickness. The 839 uses 18 jewels and 121 parts, the 849 19 jewels and 123 parts. They are otherwise virtually identical, the 849 introducing slight changes in the design of bridges. All illustrations are of the caliber 839 in a steel case.

Case engraving, backThe deceptively simple-looking case--an important element of the originality and excellence of the Ultra Thin--is comprised of three major parts (bezel, band, and back), a sapphire crystal, two gaskets, and eight screws. But the design and construction is imaginative and unusual, and provides both beauty and functional benefits. Uniquely for a watch of this thinness, the Ultra Thin is water-resistant to three atmospheres, and is rugged enough for use as an every-day watch. The case is offered in steel, red gold, and platinum.


Ultra-thin movements--roughly speaking, those under two millimeters--are notorious for delicacy, irregular running, and difficulty in servicing. There are some older Piaget ultra-thins that many watchmakers refuse to work on. The flexibility of the plate and bridges, the thinness of wheels (in the winding system and gear train), and the extremely small clearances for balance, balance spring, and pallet all create chronic alignment problems that are often nearly impossible to address. The result is poor amplitude, poor positional performance, poor daily rate, and poor reliability.Detail of center wheel

With regard to these issues, the JLC calibers 839 and 849 are sheer genius. These calibers run with the reliability, consistency, and durability of movements two or three times their thickness. How this is accomplished is, once again, in the details of design and construction, as well as the quality of manufacture. I believe that the calibers 839 and 849 are the finest ultra-thin wristwatch calibers ever produced.


My caliber 839 Ultra Thin, number 382 in steel, has provided remarkable performance for almost five years. CrownOnce a week or so, I judged it to be about a minute fast. Aside from resetting the time occasionally and winding daily, the watch has received no attention or service. The appearance of the "waterproof" crown (with internal rubber O-ring sealing on the case tube), left, suggests the kind of day-in, day-out use the watch has received. While I have never submerged the watch (and do not submerge any watch), I have treated the Ultra-Thin as I would any steel watch for daily use. Although the watch was running well, amplitude was a bit low (250 degrees, dial up), and I felt it was time for a service and adjustment. After complete disassembly, all parts were cleaned ultrasonically. I will lead the reader through reassembly of this remarkable watch.


Main plateThe bare mainplate of the caliber 839 is a bit over one millimeter thick. Excellent design, with minimal removal of metal where unnecessary, makes it surprisingly rigid. Located with sturdy alignment pins and large screws, the bridges form a box structure with the plate that is extremely rigid. At left, the lower balance pivot (1), escape wheel pivot (2), fourth wheel pivot (3), third wheel pivot (4), center wheel pivot (5), opening for setting/winding stem (6), and opening for mainspring (7) may be identified. Both the balance wheel and escape wheel are provided with complete shock protection on both ends of the pinions (pierced jewel, cap jewel, spring, and housing). Remarkably, in this thin design, the balance shock units are the same KIF units used in JLC's much thicker caliber 889, but mounted in slightly flatter assemblies. The escape wheel units are an even lower-profile KIF design. The lower center wheel pivot (also shownCenter wheel bushing right, from the dial side) is unjeweled. This is relatively common in thin movements, although it may seem peculiar to the watch collector accustomed to over-jewelled contemporary watches. While a small-diameter jewel is quite strong, a thin one is not, and is prone to cracking. Thus the use of a jewel in this position--while keeping the barrel, center wheel, and third wheel in alignment--would increase the thickness of the movement. A less durable, but stronger and replaceable, bushing (yellow arrow) is one alternative. In the caliber 849, a redesign of the barrel, train wheel, and escape wheel bridges has allowed a jewel in this position, with no increase in thickness. This is the nineteenth jewel added to the 18 of the caliber 839. The extra two parts in the 849 are a separate bridge (and bridge screw) for the escape wheel, which is included in the train-wheel bridge of the caliber 839.

Plate with barrelThe height requirement of the mainspring (for adequate driving power without excessive thickness or length, both of which demand a barrel of excessive diameter) is one of the significant design problems in a thin caliber. Thus a floating barrel is often used. This avoids the thickness of a plate below and bridge above the already tall barrel. But because the barrel pivots carry more load than any other wheel in the train, such floating arrangements often lead to pivot problems, tilting of the barrel, and poor power transmission to the center wheel.

The caliber 839 uses an extremely elegant and rigid solution to barrel mounting. An elaborated barrel arbor provides a very large, integral center bearing. The arbor mounts the barrel in the barrel bridge between the barrel and ratchet wheel (which normally rides directly on top of the barrel). The complex arbor (right) includes a Barrel arborsection internal to the barrel (1), that is attached by means of the three screws to the external section (2). This external section provides the large bearing surface that rides in the hole in the bridge (blue arrow). The ratchet wheel is attached with three screws (not illustrated) to the backside of part 2. As shown above left, the barrel actually rides "upside down" (i.e. with cover down) in the plate.

Without consistent, smooth power transmission to the rest of the wheel train, a watch cannot function properly. The large diameter bearing surface used in the calibers 839 and 849 (also bARREL ARBOR SCHEMATICillustrated in the diagram, left) is an excellent solution to the problem. Properly lubricated (with a high viscosity oil like Moebius D-5), it should provide stable performance and long life.

The illustrations below show (1) the barrel as seen from the bottom of the plate; (2) the barrel and center wheel mounted on the top of the plate; (3) the barrel bridge in position over the barrel arbor, the bearing surfaces indicated by the blue arrows; and (4) the barrel ratchet wheel attached on top of the barrel (A), and the center (B), third (C), fourth (D), and escape (E) wheels in position. Note that the jeweled center wheel upper pivot is held by the barrel bridge (F). The third, fourth, and escape wheels share a single bridge (not shown in the illustration).


Balance and pallet lever bridgeThe caliber 839 reveals several other details of design contributing to both its flatness and robustness. As illustrated right, the two spoke Glucydur balance (A) uses angled spokes (red arrow) that allow the balance wheel to be mounted very low in the plate, rise over the pallet lever bridge (B), and position the balance rim in the "channel" (C) between the pallet lever bridge and main plate. This construction provides an extremely flat, but rugged design. The arrangement also allows for better clearance between the balance spring (D) and the balance spokes below and balance cock above (blueBalance rimarrows). Interference of the balance spring is a common problem with flat movements, and absolute flatness of the balance spring--which is sometimes difficult to accomplish--is necessary in such designs. The caliber 839 is thankfully much less demanding in this regard. The balance rim in its channel running under the balance cock is also illustrated left. The 8.4 millimeter balance is unusually large for a caliber of this size, contributing to the running consistency of the watch.

One other interesting detail of the caliber is the use of two pillars, located on either side of the balance assembly, to prevent any pressure on top of the cock from the back cover of the case. Even slight pressure on top of a balance cock or shock absorber-regulatorClearance pillars assembly will affect the running of the watch, and many watchmakers have marveled at how well a thin movement runs until it is cased. Illustrated right, these pillars may be adjusted for height, and thus define the effective upper surface of the movement.


The dial side of the calibers 839 and 849 are as elegantly conceived and executed for function and flatness as the upper plate. A classic, simple, very beautiful keyless and motion works is kept as flat as possible. Yet, all Keyless and motion workscomponents remain remarkably robust. As illustrated left with the cover plate removed, the parts are exceptionally well finished. As indicated, these parts include the castle wheel (A); pull-out piece (B); return bar (C); return spring (D); intermediate wheel (E); second intermediate wheel (F); minute wheel (G); and cannon pinion (H). The dial side, with cover plate in position, is illustrated below right. Note that, as in many calibers, the pull-piece retention spring (blue arrow) is integrated into the extremely elegant cover. This spring holds the stem in discrete winding or hand-setting positions and is responsible for the resistance and detent that is felt in moving the crown from one position to the other. Keyless works with coverThe return spring (yellow arrow), maintains the stem and crown in the normal winding position (i.e. against the case). The cover is also responsible for retaining the intermediate wheel, second intermediate wheel, and minute wheel.


In addition to the assembly already illustrated, the installation of the wheel train bridge (yellow arrow, below left); balance, balance spring and balance cock (blue arrow); the transmission wheel (green arrow); the ratchet click (white arrow); and the hour wheel and dial washer (over the cannon pinion, on the dial side) complete the assembly. As illustrated, the wheel Assembled movementtrain bridge holds the upper pivots for the third, fourth and escape wheels, the latter with shock protection (red arrow). The transmission wheel transmits winding to the mainspring barrel from crown, stem, castle gear, and winding pinion (or crown gear). The ratchet click prevents unwinding of the barrel arbor, thus forcing the barrel itself to unwind and drive the center wheel.


As show right, the dial (face down, blue arrow) is attached to the assembled movement. Two dial feet (posts soldered to the back of the dial) are inserted in holes in the dial side of the plate. Movement on dial The feet are secured with screws inserted into the edge of the plate (red arrow). All markers on the dial are applied and attached with small pins protruding through the dial. The pins are soldered in place and ground flush (green arrow). This photograph also illustrates the alignment pins used on all bridges for precision of alignment and rigidity of structure (small yellow arrows).

Case partsAs with the ultra-thin caliber, rigidity and alignment are also an important issue with the ultra-thin case. Flexibility of a case does not allow proper, consistent sealing and defeats dust and water-resistant construction. A flexible case may also contribute to flexing of the movement itself. Poorly designed cases have probably been as significant a problem with ultra-thin watches as poorly designed movements. Fortunately, JLC has done a remarkably good job with the Ultra Thin case. As show left, all three major case parts are designed to contribute to rigidity. The case band is channeled for increased stiffness (blue arrow, inset) and is a single piece, without separate spacer, designed specifically for the 9 ligne caliber. Case backCase screwsThe band also has two arched channels of about 90 degrees--one each top and bottom--that correspond to protrusions on the bezel (yellow arrows). This construction locks the bezel rigidly to the band and provides a stiff, boxed construction that even enlists the rigidity of the sapphire crystal. The bezel is attached with four screws (below left, blue arrow) that extend through the back of the band forward into the bezel. The case back also attaches to the band with four, smaller, screws (red arrow) and serves to further stiffen the case. Once the movement is installed in the case, the movement plate abuts the inside band edge perfectly and is locked into place with two large screws (right). Rigidity is again enhanced. It is through such attention to design detail and quality of manufacture that JLC is able to produce a reliable, water-resistant wristwatch of such thinness.


Completed watch on dialThere are very few elements of the Master Ultra Thin that constitute new technology, or, indeed, that are even unique to this watch. In fact, with the exception of the contemporary engineering of the case, the Ultra Thin is a masterpiece of classical Swiss watchmaking. But the consistency of attention to engineering details and the extraordinarily high level of manufacturing quality have produced a whole that is much more--and, aesthetically, much less--than the combination of its parts. I believe it is fair to say that this is the finest ultra-thin wristwatch ever produced. But I would also venture that this is among the very finest watches in contemporary production, regardless of complexity. The Master Ultra Thin is probably a watch that, among Swiss manufacturers, could only have been produced by Jaeger LeCoultre. There are, in all of Switzerland, no others with the design and manufacturing skills to conceive and execute this watch with such unrelenting attention to quality. For all its simplicity of appearance, the Ultra Thin is a true masterpiece equalled by only a tiny handful of other wristwatches.

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