Omega 28

The Omega Cal. 28 family represents a foundational chapter in the brand’s history, encompassing both manual-wind and automatic variants that shaped horological development in the 1940s and early 1950s. The manual-wind Cal. 28 and Cal. 28SC (centre seconds) were robust, straightforward movements used in dress watches from 1940 onward. The Cal. 28.10 RA(introduced in 1943) marked Omega’s commercialized entry into automatic wristwatches, becoming the company’s first mass-produced self-winding movement and establishing a blueprint that evolved into the legendary Cal. 340 and subsequent 34x/35x bumper series. Collectors value these movements for their mechanical reliability, accessible serviceability, and historical significance as early-production automatics.

HISTORY & DEVELOPMENT

Manual-Wind Origins
The Omega Cal. 28 and Cal. 28SC emerged in the early 1940s as refined manual-winding movements serving the brand’s mid-range collection. These were solid, unpretentious movements designed for precision dress watches and were produced alongside the increasing sophistication of Omega’s automatic offerings.

The 28.10 RA: First Commercial Automatic
In 1943, Omega introduced the Cal. 28.10 RA PC (and shortly after, the 28.10 RA SC with centre seconds), marking the brand’s official entry into mass-produced automatic wristwatches. This was Omega’s answer to Rolex’s full-rotor system (protected by patent through much of the 1930s-1940s). Since Rolex held the full-rotor patent, Omega opted for the “bumper” automatic principle, also called a “pendulum” rotor.

The 28.10 was unidirectional wind, meaning the weighted rotor oscillated on either side and could only wind the mainspring in one direction. A spring-loaded hammer prevented the rotor from swinging beyond its mechanical limits, hence the term “bumper.” Collectors often report feeling this mechanical resistance when wearing a 28.10, which is entirely normal.

Evolution and Variants
The Cal. 28.10 was produced from 1943 to 1949 in its original form. In 1949, Omega renamed and redesignated it as Cal. 340, which then evolved into Cal. 341, 342, 343, 350, 351, 352, 353, and eventually 354, 355. These variants introduced bi-directional winding, refined bearing systems, and in later iterations (Cal. 35x), swan-neck regulators and chronometer certification. The 28.10 became the foundational architecture for Omega’s bumper automatic platform.

Notable Applications
The 28.10 RA powered early Omega Constellation models from 1952 onward (alongside newer calibers), cementing its position as an iconic movement. The Cal. 28 manual-wind variants saw use in more conservative dress watches, offering reliability without the mechanical complexity of automatic winding.

TECHNICAL DETAILS

Winding System
The manual-wind Cal. 28 and Cal. 28SC are straightforward, hand-wound movements with no automatic complications.

The Cal. 28.10 RA employs unidirectional bumper-automatic winding. The weighted rotor is a simple oscillating mass supported by a jeweled bearing on the automatic bridge. As the wearer moves their wrist, the rotor swings back and forth, engaging the ratchet wheel and winding the mainspring in a single direction only. Once the rotor reaches the limit of travel, a spring-loaded hammer arrests its motion, which causes a tactile “bump” or click sensation.

Regulation System
All Cal. 28-family movements use a fixed regulator index on the balance bridge. Adjustment requires moving the hairspring stud carrier using a regulator key or specialized tool. This is a period-appropriate system, straightforward but requiring some care during regulation. Beat error correction is possible by shifting the stud carrier position relative to the balance staff.

Construction and Finishing
These movements feature clean, utilitarian finishing typical of 1940s Omega. Plates are generally unpolarized or lightly decorated. The 28.10 RA exhibits Omega’s “OMEGA” stamp on the automatic hammer, and internal plates typically bear Omega’s caliber designation and quality marks. The movements are competently finished but lack the embellished surfaces found on higher-grade rivals of the era.

Power Reserve Performance
The 44-hour power reserve on healthy examples is genuinely impressive for 1940s design. This speaks to intelligent mainspring curve design and minimal internal friction. A properly serviced movement should deliver close to this figure.

PERFORMANCE SPECIFICATIONS

Amplitude and Baseline Behavior
Healthy Cal. 28/28.10 movements display typical amplitude in the 220-280 degree range when fully wound and freely swinging. The 28.10 RA, with its lighter rotor and single-direction wind, often shows amplitude around 220-250 degrees under normal conditions. The automatic mechanism’s friction slightly reduces the amplitude compared to manual-wind equivalents, which is normal.

Beat Error and Positional Variance
Expected beat error for these movements runs 0.0 to +2.0 ms when properly regulated. Positional variance (the rate difference between dial-up and crown-down positions) can be 10-20 seconds per day on uncompensated examples. The simple regulator design does not permit free-sprung balance compensation.

Service-Level Acceptance
In field service, watchmakers generally accept 0-10 seconds per day for dial-up position and expect variance across positions. Omega did not rate these movements to COSC chronometer standards as standard practice, though late-production 355 variants achieved chronometer certification.

REGULATION & ADJUSTMENT

Regulator Type
Fixed regulator index with hairspring stud carrier. The stud carrier can be moved along slots in the balance bridge, advancing or retarding the effective length of the hairspring, thus changing the beat rate.

Adjustment Method
A regulator key (specialized key with the correct profile) is placed over the stud carrier pin and turned slightly to move the carrier. Moving the carrier outward (toward the outer edge of the bridge) makes the hairspring effectively longer, slowing the rate. Moving inward accelerates it.

Beat Error Correction
The stud carrier can be shifted to move the hairspring attachment point, which adjusts beat error. This is less surgical than modern free-sprung systems but perfectly functional. Patience and precision are required.

Fine Adjustment Range
Approximately 10-15 seconds per day total range across the full extent of the regulator slots. This is adequate for bringing most worn or improperly serviced examples into acceptable range.

Known Quirks
The regulator index can stick or bind if contaminated with old lubricant or debris. The jewel holding the stud carrier must be clean. Once cleaned and properly lubricated, behavior is predictable and stable.

SERVICEABILITY & MAINTENANCE

Recommended Service Interval
8-10 years for movements in regular use, or immediately if the watch stops, gains/loses excessively, or has not been serviced within the last 20 years.

Parts Availability Status
Cal. 28/28SC manual-wind movements: Many parts are becoming scarce. Mainsprings, balance staffs, and hairsprings are still available from specialized suppliers (Perrin, Ofrei, WatchMaterial, some independent makers), but less common pieces (hammer springs, click springs for the 28.10) are NOS only or no longer obtainable.

Cal. 28.10 RA: Core parts (mainspring, barrel arbor, balance) are obtainable. The unique bumper rotor assembly and automatic bridge can be found as NOS, but expect to pay premium prices. Service-replacement parts (non-original but functional) are sometimes available.

Typical Service Costs
Basic service (cleaning, re-oiling, regulation): $200-400
Full overhaul (disassembly, parts replacement as needed, refinishing if applicable): $400-700
Prices vary regionally and depend on movement condition.

Required Specialty Tools

• Movement holder appropriate for 28mm-diameter movements
• Regulator key (varies by maker; improvisation is sometimes necessary)
• Pin vises and stakes for balance wheel and pallet removal
• Standard watchmaker screwdrivers
• Jeweler’s loupe (10x minimum)
• Timing machine or oscilloscope (modern shops)

Recommended Lubricants (by position)

Critical note for the 28.10 RA: Over-lubrication of the rotor bearing will reduce winding efficiency. Use minimal quantities. The rotor should swing freely with a light push, not lag or stop abruptly.

KNOWN ISSUES & FAILURE POINTS

Cal. 28/28SC Manual-Wind:

• Mainspring brittle or broken: Age-related. Original springs from the 1940s-1950s sometimes crystallize. Always replace with modern equivalent.
• Balance staff worn or bent: The balance pivot is a high-stress point. Worn pivots cause amplitude loss and erratic timekeeping.
• Click spring worn or missing: The click prevents the ratchet wheel from reversing. A weak click allows the mainspring to unwind, causing power loss.
• Hairspring kinks or coils stuck together: Improper storage or previous rough service. Hairspring replacement is often necessary.
• Worn pallet stones: Excessive friction between pallet and escape wheel teeth wears the ruby or synthetic stones, causing hesitation and inaccuracy.

Cal. 28.10 RA Bumper Automatic:

• Weak or broken bumper spring: The spring-loaded hammer prevents the rotor from over-extending. If weak or broken, the rotor strikes the bridge hard, causing damage and vibration felt by the wearer. Replacement is difficult; often, the entire automatic bridge must be sourced.
• Rotor bearing wear: Jeweled bearings (usually ruby) can wear, allowing the rotor to wobble. This reduces winding efficiency. Observable under magnification.
• Ratchet wheel teeth worn or chipped: The rotor engages the ratchet wheel. Worn teeth slip, preventing winding. Rare but seen in heavily used examples.
• Clicking sound when the rotor reaches limit: This is normal behavior, not a defect. The bumper spring is designed to do this.
• Slow or no automatic winding: Check for: broken bumper spring, worn rotor bearing, inadequate lubrication, or bent rotor. The rotor must swing freely.
• Hairspring issues: Same as manual-wind variants; age and previous service history affect reliability.

Design Weak Points for Inspection

• Pivot condition on the balance staff: Always check for bending, wear, or corrosion. Bent pivots cause wobble and amplitude loss.
• Jewel settings on the balance bridge: Over-tightening or previous rough service can crack jewels. Inspect under magnification.
• Ratchet wheel and click spring engagement: The click must seat firmly and return smoothly. Worn teeth or springs lose holding power.
• Hairspring collet: If cracked or loose, the hairspring detaches partially, causing erratic behavior.
• Automatic rotor assembly on 28.10 RA: Look for cracks in the bridge, wear on the jewel bearing, and verify the bumper spring functions. Cracks in the bridge are not repairable.

Signs of Improper Previous Service

• Polished or stripped screws: Indicates forceful or careless disassembly. Check for crossed threads or broken screws left inside.
• Replacement balance wheel that does not match original: Period-correct replacements are sought by collectors. Non-original balances affect performance and value.
• Mismatched jewels or crude jewel settings: Poor-quality repairs. Inspect the quality of the stone fit and setting.
• Over-oiled pivot holes or sticky pallet stones: Previous service used wrong lubricant or quantity. This must be cleaned thoroughly.
• Replaced click spring or ratchet wheel (28.10 RA): Not always obvious; compare part shapes and finish against reference examples or caliber diagrams.

Age-Related Wear Patterns

• Oxidation on copper-alloy wheels: Normal after 70+ years. Copper wheels may show green patina or brown discoloration. This does not affect function if pivots remain sharp.
• Dulled or thin pivot tips: Normal wear. Sharp pivots are ideal; dull or worn pivots increase friction and reduce amplitude.
• Loose center wheel or gear: Wear on the arbor or pinion can allow wheels to rock slightly. This shows up as variation in beat error across positions.
• Hairspring colored or discolored: Age-related. Does not always indicate failure, but discolored springs should be tested carefully and often replaced.

PARTS INFORMATION & DIAGRAMS

Key Replacement Parts

Parts Compatible with Related Calibers
Manual-wind Cal. 28/28SC shares some parts with other Omega manual-wind movements of the 1940s (Cal. 265, 266), but differences are significant. Cross-compatibility is limited.

The Cal. 28.10 RA shares architecture with its successor, Cal. 340 (1949+), but the designs diverged. The automatic bridge and rotor system of the 28.10 are not interchangeable with Cal. 340-355 variants.

Commonly Worn or Replaced Components

• Mainspring: Brittleness and loss of power are typical failures.
• Balance staff: Wear or bending is common.
• Click spring: Weakens over time, allowing mainspring to slip.
• Pallet stones: Wear from friction with escape wheel teeth.
• Hairspring: Kinks, collapses, or sticking coils develop with age.

eBay Search Links for Parts

Search for “Omega 28 parts” on eBay

Search for “Omega vintage movement parts” on eBay

Search for “Omega mainspring vintage” on eBay

Note: Parts availability is unpredictable. Most searches yield complete movements rather than individual components. Expect to source many parts through established watch parts suppliers like Perrin, Ofrei, or WatchMaterial rather than eBay alone.

COMPATIBILITY

Dial Foot Positions and Spacing
Cal. 28 movements use standard 28mm-diameter mounting. Dial feet are typically positioned at 12 and 6 o’clock, spaced to fit Omega’s case rings of the era. Spacing varies slightly between watch references; always measure and verify against the original dial before assuming compatibility.

Hand Sizes

• Hour pipe diameter: Typically 4.0 mm (0.157″)
• Minute pipe diameter: Typically 3.6 mm (0.142″)
• Seconds pinion: Typically 2.8 mm (0.110″)

Hand fitting requires precision; oversized hands risk damage during installation, undersized hands slip. Modern watch repair shops can resize hand holes with care.

Case Compatibility
Cal. 28/28.10 movements fit Omega case references with a minimum internal diameter of approximately 28.5-29.0 mm. The movement height of 3.25 mm is compact, fitting well into case backs of the era (typical depth 10-12 mm). Constellation and Seamaster models of 1952-1955 used these movements, as did numerous dress watch references.

Stem and Crown Compatibility
The movement requires a tap size 6.75 mm (Omega standard of the era). Stem length varies by case design but typically 6.5-7.5 mm from the movement to the case back. Rolex-compatible crowns sometimes adapted, though fit and finish may not be perfect.

Crystal and Movement Ring Requirements
These movements used plexi-glass (acrylic) crystals typically 30-34 mm in diameter, depending on case reference. Movement rings (the spacer between case and movement) vary by case. Always verify original specifications before ordering replacements.

IDENTIFICATION & MARKINGS

Locating the Caliber Number
The caliber number is engraved on the top plate (visible when the balance is removed) and often stamped on the automatic bridge of Cal. 28.10 RA movements. Look for “Cal. 28,” “Cal. 28SC,” “Cal. 28.10 RA PC,” or “Cal. 28.10 RA SC.”

Common Engravings and Stamps

• “OMEGA” logo: Stamped on the movement plate and on the rotor hammer (28.10 RA only).
• Caliber designation: Engraved on top plate.
• Quality marks: Omega used proprietary symbols indicating quality grade or finishing level, though detailed documentation is limited.
• Service marks: If previously serviced by Omega, faint stamps or marks may appear (varies by era).

Distinguishing from Similar Calibers
Cal. 28 vs. Cal. 28SC: The 28SC has an additional sweep seconds hand and complication. Visible on the dial; 28SC has a sub-second dial, 28 does not.

Cal. 28.10 RA vs. Cal. 340: The 28.10 RA is unidirectional wind (single-direction winding). The rotor is a simple weighted disc. Cal. 340 (introduced 1949) has a bi-directional rotor, a more efficient automatic bridge, and refined bearing systems. The bridges are visually different.

Manual-wind 28 vs. automatic 28.10 RA: Look for the automatic bridge and rotor. Manual-wind Cal. 28 has no automatic mechanism; the movement appears cleaner and less complex from the back.

Signs of Service or Modification

• Replaced balance wheel: Non-original balances may show different engraving, wear patterns, or wire configuration. Compare against documented examples.
• Mismatched screws: Original Omega screws have specific slots and finish. Replacement or repaired screws stand out.
• Refinished plates: If the movement was polished (a poor practice), plates lose their original patina and finish uniformity. Inspect under magnification.
• Non-original jewels: Crude setting or stone that doesn’t match the period’s standards indicates repair by lesser craftsmen.
• Reassembled incorrectly: If the automatic bridge is loose or binding, the movement was likely reassembled improperly.

COLLECTOR CONSIDERATIONS

Value Drivers

• Original finish on dial and movement: Unpolished plates and original dial finish command premium prices.
• Factory stamps and quality marks: Rare variants with chronometer stamps or special markings add value.
• Condition of the automatic rotor (28.10 RA): A complete, undamaged rotor assembly and bumper spring are highly desirable.
• Matched serial numbers: Watches where the case, dial, and movement serial numbers align are valued higher than mixed assemblies.
• Rarity of reference: Some early Constellation or Seamaster models or limited-production dress watches command premium valuations.
• Radium dial markers (1940s examples): Original radium lume is historically significant, though hazardous. Discloses honestly in sales.

Red Flags to Watch For

• Polished movement plates: This is the primary killer of value. Original patina cannot be restored.
• Replaced balance wheel: Verify originality via service marks or comparison to documented examples. Non-original balances reduce value 10-20%.
• Aftermarket screws: Replacement screws indicate amateur service history.
• Worn or damaged bumper spring (28.10 RA): This component is expensive or impossible to replace, making the automatic system unreliable.
• Cracked automatic bridge: Non-repairable; signals a significantly compromised movement.
• Mismatched dial and case: Serial numbers should generally align or at least be from the same era. Frankenwatches are disclosed and undervalued.

Known Reproductions or Franken-Parts
Reproductions of Cal. 28 movements are rare; the market is too small to justify counterfeiting. However, mixed or “Franken” watches are common: a Cal. 28 movement in a non-original case, or a later Cal. 340+ movement misrepresented as a 28.10. Always verify the movement engraving and rotor style if purchasing.

Originality Assessment
What can be replaced without harming value:

• Mainspring (modern equivalent is standard)
• Hairspring (if original is damaged)
• Damaged jewels in settings
• Worn pallet stones
• Non-visible components (barrel arbor wear)

What should remain original:

• Balance wheel (period-correct replacement is acceptable; non-original reduces value)
• Hairspring (if original is acceptable, preserve it)
• Ratchet wheel and automatic bridge (28.10 RA)
• Movement plates and bridges (refinishing destroys value)
• Screws (original Omega screws are preferred)
• Rotor (Cal. 28.10 RA; irreplaceable)

A well-serviced movement with period-correct parts is more desirable than a pristine but unserviced movement that will fail imminently.

REFERENCE MATERIALS

Technical Documentation
Omega’s official service documentation for Cal. 28 and Cal. 28.10 RA is increasingly scarce. Some resources:

• Caliber diagrams and exploded views: Occasionally found on specialist forums or in PDF archives via Scribd, archive.org, or horological societies. Search “Omega caliber 28 manual filetype:pdf.”
• Parts catalogs: Vintage Omega parts lists from the 1940s-1950s are available through specialized dealers and online archives.
• Service notes: Watchmaker blogs and forums (WatchRepairTalk, OmegaForums) contain field notes and service experiences.

Recommended Books

• “Omega Wristwatches” by Jaquet and Chapuis: Comprehensive Omega history with caliber diagrams.
• “The Best Wristwatches in the World” by Kahlert, Mühe, and Brunner: Includes Omega caliber specifications.
• “Vintage Omega Wrist Watches” by various enthusiast authors (found in watch collector forums and sites like Omegamania): Focused entirely on Omega vintage models and movements.

Archives and Forums

• OmegaForums.net: Large community with documented Cal. 28 and 28.10 threads, technical discussions, and service photos.
• WatchRepairTalk.com: Field experience from professional watchmakers; search “Omega 28” for relevant discussions.
• Ranfft.de: Comprehensive caliber database (Ranfft) with specs and technical drawings for Cal. 28 and variants.
• Omegamania.com: Collector database with reference numbers, production dates, and movement specifications.

Parts Catalogs and Exploded Diagrams
Vintage Omega parts catalogs (1940s-1950s) are occasionally found as PDF scans. Exploded diagrams for Cal. 28/28.10 exist in some technical archives; watchmaker supply houses (Ofrei, Perrin, SoFly) maintain reference materials.

RELATED CALIBERS

The Cal. 28 family evolved into Omega’s bumper automatic platform, which dominated the 1950s before the full-rotor and Co-Axial systems gained prominence.

SUMMARY

The Omega Caliber 28 family stands as a defining movement family from Omega’s formative automatic era. The manual-wind variants (Cal. 28, 28SC) represent clean, functional watchmaking typical of 1940s manufacturing. The Cal. 28.10 RA, Omega’s first commercial automatic, introduced the bumper principle and proved durable enough to remain in active service today.

These movements are collector favorites not because they represent cutting-edge engineering, but because they embody mechanical clarity and proven longevity. A properly serviced example will perform reliably for decades more. The primary challenge remains sourcing quality replacements for scarce components like the bumper spring (28.10 RA) and worn balance staffs.

For watchmakers and collectors alike, the Cal. 28 family rewards careful stewardship and honest service. Invest in proper overhaul, respect the original finish, and these humble movements will outlast trends.