1. Historical Background of Felsa 10s Movements
Felsa SA, founded in 1918 in Lengnau, Switzerland, holds a revered position in the annals of Swiss horology. While the manufacturer gained modern fame for pioneering the bidirectional automatic rotor with the Bidynator movement in the 1940s, its early foundation was built upon producing exceptionally reliable pocket watch ébauches during the 1910s and 1920s. Among these early mechanical platforms, the Felsa 10-size movement family stands out as a triumph of interwar engineering, bridging the gap between classical 19th-century pocket watch construction and the emerging mid-century demand for compact wrist-worn timing instruments.
During the early decades of the 20th century, Swiss watchmaking was undergoing a fundamental paradigm shift. Pocket watches were gradually ceding ground to wristwatches, initially spurred by military necessities during World War I. Felsa capitalized on this transition by designing movement architectures that maintained the structural robustness of pocket watch calibers—such as wide bridge spacing, oversized mainspring barrels, and heavy-duty gear trains—while paring down overall dimensions. The 10-size designation (often aligning with 10½ ligne or American 10-size scale measurements depending on regional casing specifications) became a popular middle ground for horologists searching for high accuracy in a manageable form factor.
Today, these historical Felsa movements are prized by modern restoration watchmakers. Their high build quality, clean bridge layouts, and resilient wheel trains make them ideal candidates for horological preservation projects. Retaining their historic value while giving them a new life on the wrist requires deep mechanical appreciation, precise machining, and a strict adherence to period-correct watchmaking principles.
2. Technical Specifications and Movement Architecture
Mechanically, the Felsa 10s movement platform features a classical Swiss lever escapement layout running at a traditional frequency of 18,000 vibrations per hour (2.5 Hz). This leisurely beat rate provides the characteristic long, resonance-rich tick associated with vintage pocket watches while minimizing wear across the pallet jewel faces and escape wheel teeth. The escapement is typically paired with a cut bimetallic compensation balance wheel featuring steel-and-brass rims and solid timing screws, or in later iterations, a mono-metallic balance spring coupled with a flat hairspring or a delicate Breguet overcoil spring.
The movement plate and bridge architecture typically utilize a three-quarter plate design or a series of finger bridges that showcase meticulous Swiss finishing techniques. Depending on the grade, Felsa 10s movements were produced in variations ranging from basic 7-jewel configurations up to fully adjusted 15 to 17-jewel variants featuring screwed gold chatons and polished counter-sinks. The center wheel, third wheel, and fourth wheel are constructed from high-grade hard-drawn brass, driving steel pinions with burnished pivots resting in synthetic ruby jewel bearings.
The keyless works on the Felsa 10s are engineered with a traditional sliding pinion and setting lever arrangement (tirette system). Depending on whether the ébauche was intended for an open-face (Lépine) or hunter (Savonnette) case layout, the crown wheel and ratchet wheel sit flush against the barrel bridge, secured by left-hand threaded retaining screws. The mainspring barrel houses a thick, high-torque carbon steel or early alloy mainspring capable of delivering a power reserve of approximately 32 to 38 hours, providing ample force to drive the heavy handset across the dial plane.
3. Challenges of Converting Felsa 10s to Wristwatch
When enthusiasts ask: can you convert felsa 10s pocket watch movements into reliable daily timepieces, the answer is yes, but it presents unique watchmaking hurdles that demand meticulous intervention. The primary challenge lies in the fundamental layout differences between pocket watch architectures and modern wrist-worn orientations. In an open-face (Lépine) Felsa 10s movement, the winding stem is positioned directly inline with the sub-seconds arbor at the 12 o'clock and 6 o'clock axis. Rotating the movement inside a wrist case so that the crown sits at the standard 3 o'clock position shifts the sub-seconds dial to an asymmetrical 9 o'clock position—a layout that requires specialized dial considerations or custom hand tracking.
Another critical concern is shock protection. Felsa 10s pocket watch movements were engineered long before the invention of modern shock-absorbing jewel settings such as Incabloc or KIF. The delicate balance pivots—often measuring less than 0.09 millimeters in diameter—run directly inside fixed balance jewels. On a pocket watch resting in a vest pocket, these pivots face minimal lateral stress; on a wrist, however, sudden impacts against hard surfaces can easily snap an unshielded balance staff. Converting these movements requires careful adjustment of pivot endshake and side-shake, perfect jewel alignment, and clear advisory to the wearer regarding daily activity limitations.
Furthermore, dust and moisture resistance present persistent risks. Pocket watch cases were never designed to withstand the humidity, sweat, and environmental exposure that wristwatches encounter daily. Achieving a secure fit inside modern casing assemblies without compromising the delicate movement plates demands precision tolerances. For those interested in seeing how historical movements are restored across various eras, exploring antique pocket watch conversions offers valuable insight into how master watchmakers balance historical preservation with functional durability.
4. Lug Welding and Case Selection
The foundation of any successful conversion project rests upon selecting or fabricating a suitable felsa 10s pocket watch to wrist conversion case. Watchmakers generally have two paths: modifying an original period pocket watch case by attaching custom wire lugs or fabricating/adapting a modern CNC-machined 316L stainless steel wristwatch case designed specifically for vintage movement diameters.
If modifying an original brass, silver, or gold-filled period case, silver soldering or modern micro-laser welding is required to attach sturdy lugs. Wire lugs, popular during the trench watch era, can be formed from solid sterling silver or stainless steel wire and welded directly to the case body at 12 o'clock and 6 o'clock. Extreme heat management is vital during silver soldering; the case tube, bezel, and case back must be stripped of all solder-sensitive components, crystals, and gaskets to prevent thermal warping of the hinge assemblies or bezel threads.
Alternatively, using a modern custom conversion case provides superior structural protection and everyday wearability. These cases feature scratch-resistant sapphire crystals, screw-down display backs, and high-tensile rubber O-ring gaskets. Because pocket watch movements lack modern case screws in standard positions, the watchmaker must lathe-turn a custom internal movement spacer ring—typically from marine-grade brass or 316L steel—that grips the Felsa 10s pillar plate around its circumference, securing it perfectly concentric within the case frame without putting strain on the balance cock or bridge edges.
5. Crown and Stem Solutions
Engineering a reliable winding and setting mechanism is often the most labor-intensive step when you convert felsa 10s pocket watch into wristwatch configurations. Pocket watches traditionally utilized long winding arbors housed inside heavy pendant tubes, often working in tandem with negative-set sleeve mechanisms built into the pocket watch case itself. Modern wrist cases, by contrast, rely on positive-set winding stems that pass through a short, gasket-sealed pendant tube into the keyless works.
To overcome this discrepancy, the watchmaker must often machine a custom stem on a watchmaker's lathe (such as a Bergeon or Schaublin lathe). Starting with annealed high-carbon steel rod stock, the maker turns the pivot stem diameter, files the square winding section to fit the Felsa sliding pinion, and cuts the precise setting lever groove. Once test-fitted, the stem is thread-cut to match modern crown taps (typically Tap 8 or Tap 9) and subsequently heat-treated, quenched in oil, and tempered to a straw color to achieve the requisite mechanical strength without brittleness.
The crown selection must harmonize historically with the case while providing an effective moisture barrier. A onion-style or pumpkin-style crown is historically coherent for transitional watch styles, but it must be fitted with an internal double O-ring gasket that seats tightly over the case pendant tube. This ensures that when the user winds the high-tension mainspring of the Felsa movement, no lateral flex is transmitted to the setting lever screw or keyless works yoke.
6. Dial and Hand Considerations
The dial is the soul of any horological conversion. Original Felsa 10s movements frequently feature exquisite grand feu enamel dials or silvered brass dials with hand-engraved numerals. Preserving an original vitreous enamel dial requires immense care; attempting to modify dial feet locations on fired enamel will almost certainly cause spider-web fractures across the porcelain surface.
If the original dial feet do not align with the conversion case orientation or if the dial feet have sheared off over time, traditional watchmakers avoid adhesives or temporary dots. Instead, micro-welding new copper dial feet using a capacitive discharge dial foot welder allows precise attachment without overheating the dial face. If the movement layout places the sub-seconds at an unorthodox angle (such as 9 o'clock), the dial must be oriented carefully to ensure the main hour index remains perfectly vertical relative to the 12 o'clock lug position.
Hand fitting presents another mechanical task. The arbors of early Felsa pocket movements—specifically the center wheel cannon pinion, hour wheel pipe, and fourth wheel pivot—often feature dimensions distinct from modern wristwatch standards. Vintage steel hands must be selected, annealed, and carefully re-reamed or staked using a staking set to fit the original Felsa arbor diameters precisely. Bluing the hands over a traditional brass shaving pan heated by a flame yields a rich, cornflower blue thermal oxidation layer that honors period aesthetics, reminiscent of those seen on authentic WW1 trench watches.
7. Why 10s is More Common for Conversion
Within the conversion community, movement sizing plays a decisive role in the ultimate wearability of the finished watch. Large American railroad pocket watch movements, such as 16-size (approx. 43mm movement diameter) or 18-size (approx. 45mm movement diameter), yield finished wristwatches that measure 48mm to 52mm across the case. These oversized dimensions often prove uncomfortably large for daily wrist wear and are prone to catching on sleeve cuffs and suffering impact damage.
The 10s movement platform strikes the ultimate mechanical balance. With movement diameters typically ranging between 38mm and 40mm, encasing a Felsa 10s results in a finished wristwatch measuring between 41mm and 44mm. This size falls precisely into the modern sport watch aesthetic—offering commanding wrist presence while maintaining comfortable weight distribution and ergonomic lug curvature.
Moreover, the Felsa 10s ébauche was produced in generous quantities, meaning spare components such as balance shafts, escape wheels, and mainspring barrels remain accessible to master watchmakers. The overall mechanical architecture is robust enough to tolerate full restoration, while being compact enough to fit into custom modern cases without requiring excessively thick profiles.
8. Final Thoughts on Felsa 10s Conversions
Converting a historical Felsa 10s movement from a pocket watch into a custom wristwatch is far more than a simple mechanical re-casing; it is an act of horological preservation that honors early 20th-century Swiss manufacturing. By translating a movement designed for a vest pocket into an instrument built for the wrist, watchmakers bridge two distinct eras of micro-engineering.
Whether you choose to utilize an original restored metal case with hand-welded lugs or opt for a modern CNC-machined stainless steel housing with sapphire crystal, success hinges on uncompromising craftsmanship. Perfect keyless works alignment, precision lathe stem machining, proper shock management, and sensitive dial preservation ensure that the legendary Felsa ébauche continues to tick reliably for generations to come.