ST1903 - movement

The Sea-Gull ST1903 is a mechanical chronograph watch movement manufactured by the Tianjin Sea-Gull Watch Corporation in China. [1, 2]

History & Heritage

Swiss Venus 175 Roots: In the 1960s, the Tianjin Watch Factory acquired the original machinery and blueprints of the Swiss Venus Calibre 175 movement (part of "Project 304") to produce aviation chronographs for the Chinese Air Force. [1, 2]
Resurrection: The Tianjin Sea-Gull Corporation later revived and modernized the platform in the 2000s as the ST19 family. Users on Reddit generally agree that its heritage as a direct Venus clone gives it unique mechanical value. [1, 2, 3]

Key Specifications & Features

Type: Manual-winding, column-wheel mechanical chronograph.
Layout (ST1903): A 3-register configuration featuring a 30-minute chronograph counter, a small seconds sub-dial at 9 o'clock, and a 24-hour indicator at 6 o'clock (synchronized with the main time).
Frequency: 21,600 vibrations per hour (3 Hz).
Jewels: Typically 22 jewels (with slight variations across the wider ST19 series spanning 19–23 jewels).
Power Reserve: Approximately 40 to 51 hours.
Diameter & Height: 31.3 mm diameter and roughly 6.25 mm thick. [1, 2, 3, 4]
Column-wheel and horizontal clutch. It pairs a traditional column wheel with a lateral (horizontal) coupling clutch rather than a modern vertical clutch.
Visible mechanical action. Because it uses a horizontal clutch, you can see the intermediate coupling wheel physically swing over to mesh with the central chronograph wheel when you press the start pusher through a display caseback.

Market Position

The ST1903 (and its siblings like the ST1901) is widely praised as one of the most affordable entry-level mechanical column-wheel chronographs available in modern watchmaking, frequently found in vintage-inspired reissues and microbrand timepieces

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are the st1901 and st1903 movements the same size?

Yes, the Sea-Gull ST1901 and ST1903 movements are the exact same physical size, sharing a 31.3 mm diameter (13.75 ligne) and a 6.25 mm overall height. [1, 2, 3]

Because both calibers are built on the same Tianjin Sea-Gull ST19 (Venus 175) base architecture, they share identical case-fitting dimensions, stem heights, and dial-foot placements. [1, 2, 3]

Key Dimensions & Differences

Specification	Sea-Gull ST1901	Sea-Gull ST1903
Casing Diameter	31.3 mm (31.0 mm fitting)	31.3 mm (31.0 mm fitting)
Total Height	6.25 mm (5.85 mm without dial plate)	6.25 mm (5.85 mm without dial plate)
Stem Height	1.8 mm	1.8 mm
Subdial Layout	2 registers: Small seconds at 9:00, 30-minute counter at 3:00	3 registers: Small seconds at 9:00, 30-minute counter at 3:00, and a 24-hour indicator at 6:00

Swapping & Compatibility Note

While the movements fit into the exact same watch cases, the dials are not directly interchangeable without modification. The ST1903 adds a gear train and pinion at the 6 o'clock position for the 24-hour hand. Putting an ST1901 two-register dial over an ST1903 movement requires removing the 6 o'clock cannon/hour wheel or drilling a hole in the dial to clear the pinion

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The Sea-Gull ST1903 (and the broader ST19 family) has a movement diameter of 31.3 mm and a thickness of 6.25 mm. [1]

Because the movement itself is 31.3 mm wide, you need a case specifically engineered for the ST19 series (often shared with ST1901/ST1902 movements). Depending on the exterior style you want, completed watch cases typically range from 38 mm to 43 mm in outer diameter: [1, 2]

38 mm Cases: (such as Pre-Daytona styles) fit dials around 30.5 mm to 32 mm and have an overall case thickness of roughly 15.5 mm. Find options via Alpha Europe. [1]
42 mm Cases: (such as Daytona styles) feature a 40 mm bezel diameter, a 32 mm case inside diameter, and a thickness around 16 mm. Find options via Alpha Europe. [1]
43 mm Cases: (such as Speedmaster styles) offer a 40 mm bezel, 33 mm inside diameter, and ~16 mm thickness. Find options via Alpha Europe. [1]

Key Things to Check Before Buying:

Movement Diameter Match: The case must accommodate the 31.3 mm caliber and match the stem height/position of the ST19 series. Do not try to force an ST19 into a case built for quartz movements like the VK63 (which has a 30.8 mm diameter and different stem alignment). [1, 2]
Dial Size: Make sure your dial outer diameter matches the case's internal shoulder/dial seat (typically 29 mm to 33 mm depending on the case model). Note that the ST1903 has a specific subdial layout (including a 24-hour hand at 6 o'clock and small seconds at 9 o'clock), so your dial must match this exact chronograph register layout. [1, 2, 3, 4]
Kit Inclusions: Buy a complete case set that includes the case, crystal, case back, crown, pushers, and movement/spacer ring so you don't run into hand-clearance or stem-length issues. [1]

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