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Zenith Chronomaster Open 39.5mm Automatic Mens Watch
Ref. 03.3300.3604/21.C822
The Chronomaster Open carries one of the more consequential movements in watchmaking history. The El Primero debuted in 1969 as the first automatic chronograph calibre to beat at the high frequency that genuine 1/10th-second reading requires, and the 3604 is its most considered evolution: a silicon escapement wheel shaped as a Zenith star runs without lubrication, hacking seconds have been added where the original architecture offered none, and an open baseplate between nine and eleven o'clock puts the balance wheel and purple-blue pallet lever directly in view while the watch is running. The chronograph seconds hand sweeps a full revolution in ten seconds rather than sixty, which is how the fraction becomes legible rather than estimated. The shift from the older 42mm generation to 39.5mm is not cosmetic. At 45.2mm lug-to-lug, the case sits flat against the wrist in a way the previous proportions could not manage on a typical wrist, and the downward-curved lugs and smooth bezel let it pass under a cuff cleanly. The tricolour sub-dial scheme — grey and blue counters, with a clear disc carrying the running seconds hand to preserve symmetry over the aperture — keeps the dial readable despite everything happening behind it. The Cordura-effect rubber strap bridges that contrast between mechanical complexity and wearable practicality without making a fuss of it.
- Reference
- 03.3300.3604/21.C822
- Brand
- Zenith
- Collection
- Chronomaster
- Case size
- 39.5mm
- Case thickness
- 13.6mm
- Lug width
- 20mm
- Case material
- Stainless steel
- Case shape
- Round
- Dial
- Black
- Crystal
- Sapphire
- Movement
- Automatic
- Calibre
- El Primero 3604
- Functions
- Chronograph, Small seconds
- Water resistance
- 100m
- Bracelet / strap
- Rubber strap
- Strap colour
- Black
- Clasp
- Deployant clasp
- Lume
- Luminous
- Condition
- Ex-display
- Retailer
- Goldsmiths
Additional specifications
- Lug-to-lug
- 45.2mm
- Power reserve
- 60 hours
Learn more about Zenith
Georges Favre-Jacot founded Zenith in Le Locle in 1865, aged twenty-two, with an idea that was radical for the Swiss industry: bring every trade needed to make a watch — movement, case, dial, hands — under one roof and one management. The integrated manufacture was largely his invention, and the Le Locle site he built is still in use.
The approach produced results at observatory trials, where Zenith accumulated more precision awards than any other manufacture. The name itself came from a calibre Favre-Jacot considered the summit of his work.
Its defining achievement is the El Primero of 1969, developed over seven years and announced in the same year as the Heuer-Breitling consortium's Calibre 11 and Seiko's 6139. It was an integrated automatic chronograph beating at 36,000 vibrations per hour, allowing measurement to a tenth of a second — a frequency almost nothing else matched for decades.
It nearly did not survive. When American owners ordered the mechanical tooling scrapped during the quartz crisis, a watchmaker named Charles Vermot hid the plans, presses, and cams in a walled-off attic. When demand for mechanical chronographs returned, Zenith could restart production because of him. Rolex used the El Primero in the Daytona from 1988 to 2000.
LVMH acquired Zenith in 1999.
- 1865
- Georges Favre-Jacot founds the integrated manufacture at Le Locle.
- 1969
- The El Primero automatic chronograph debuts at 36,000 vph.
- 1975
- Charles Vermot hides the El Primero tooling from destruction.
- 1988
- Rolex adopts the El Primero for the Daytona.
- 1999
- Acquired by LVMH.
Also in 6 collections
This watch appears in the Zenith Chronomaster range (19), Back in Stock (4,847), Stainless Steel Watches (3,197), Automatic Watches (1,736), Chronograph Watches (1,234) and Pre-Owned, Ex-Display & Vintage Watches (344).
Manufacturer’s guarantee
What the specifications mean
A mechanical watch for everyday wear.
Strengths
- 100m water resistance covers swimming and watersports
- 39.5mm suits most wrists
- 45.2mm lug-to-lug keeps the 39.5mm case wearable
- Sapphire crystal, which resists scratching
- Luminous hands and markers for low light
Zenith Chronomaster Open alternatives
Watches from other brands at a similar price, style and size, so you can see what else your budget can buy.
| Specification | 03.3300.3604/21.C822 | 103481 | Q9008480 |
|---|---|---|---|
| Price | £6,250 | £6,415 | £6,050 |
| Case size | 39.5mm | 41mm | 41mm |
| Water resistance | 100m | 100m | 100m |
| Movement | Automatic | Automatic | Automatic |
| Strap | Rubber strap | Stainless steel bracelet | Leather strap |
| Power reserve | 60 hours | 42 hours | 40 hours |
| This watch |
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Your new watch
Frequently asked questions, and things you should know about this watch
Can I swim with it?
Yes. At 100m it is rated for swimming and snorkelling, though not for diving.
Water resistance explainedWill the glass scratch?
Unlikely. Sapphire is second only to diamond in hardness and shrugs off everyday marking.
Sapphire, mineral and acrylic comparedDoes it need a battery?
No. It is automatic — the motion of your wrist winds it, and there is no cell to replace.
Quartz, automatic and solar comparedHow long does it run for when it's not worn?
About 60 hours fully wound — enough to survive a night off the wrist, but not a full weekend.
What size strap does it take?
20mm. Any strap of that width will fit, and on most modern watches quick-release bars make the swap a two-minute job with no tools.
Will it fit my wrist?
Lug-to-lug is 45.2mm, which suits wrists from roughly 16cm (6¼ inches) upward.
Lug-to-lug decides fit more than case diameter does.
How to size a watch to your wristIs it a men's or women's watch?
Listed as men's, though at 39.5mm it sits in the range that suits anyone.
Cheapest place to buy
Goldsmiths is the only UK retailer we track stocking this reference, so there is nothing to compare it against today.
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