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
, 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
) 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
). 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 (blue<br “Balance rim”>arrows). 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


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
); 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
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.

© 2012 Bourne In Time Inc.