Tianjin Sea-Gull automatic (ST) mechanical movements

 

Tianjin Sea-Gull automatic (self-winding) mechanical movements are generally organized and released in the following chronological sequence of development by the Tianjin Watch Factory / Sea-Gull Group: [1]

• Sea-Gull ST16 (introduced c. 1980s / early development era):
  • Features a Miyota-inspired simplified layout with a Seiko-style auto-winding rotor mechanism.
  • Runs at 21,600 vph with a ~36–39 hour power reserve and forms the basis for numerous multi-calendar and 24-hour variants (such as the ST1612). [1, 2]
Sea-Gull ST17 (introduced as an evolution of the ST16):
• Refined and upgraded architecture over the ST16, offering cleaner spacing, small-seconds, jump-hour, big-date, and power-reserve variants.
• Runs at 21,600 vph with a ~39-hour power reserve. [1, 2, 3]
Sea-Gull ST18 (introduced as a high-end thin movement):
• Modeled after the slim, high-beat Swiss ETA 2892-A2 architecture.
• Features a high-beat rate of 28,800 vph, around 172 components, a Swiss Incabloc shock system, and a ~42-hour power reserve. [1, 2, 3, 4]
Sea-Gull ST21 / ST2130 (introduced as a high-beat workhorse):
• A direct, high-frequency clone of the Swiss ETA 2824-2 (often informally referenced in early circles as the ST24).
• Operates at 28,800 vph (8 beats per second) with 28 jewels and a ~39–42 hour power reserve. [1, 2, 3, 4]
Sea-Gull ST22 (introduced as a twin-movement/modular concept):
• Features a unique configuration utilizing a pair of smaller ST6 automatic movement plates side-by-side or stacked on a common mainplate with open-heart or big-date options. [1]
Sea-Gull ST25 (introduced as a heavy-duty/complication platform):
• A larger, robust automatic family often built to host complex additions like power-reserve indicators, dual time zones, open-heart setups, and floating/suspended tourbillons running at 21,600 vph with a ~45-hour power reserve. [1, 2, 3]

(Note: The famous manual-wind Sea-Gull ST19 chronograph originates separately from the time line above. The ST19 design is descended from the Swiss Venus 175 blueprints during Project 304 in the early 1960s, rather than belonging to the main self-winding ST-series automatic timeline above.) [1, 2, 3]

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ST1701 Movement

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The Sea-Gull ST1701 (also known as TY2706) is a reliable, Chinese-made automatic mechanical watch movement featuring a sub-dial for small seconds at the 6 o'clock position. [1, 2, 3, 4]

Key Technical Specifications

Manufacturer: Tianjin Sea-Gull [1]
Type: Automatic with manual hand-winding [1]
Jewels: 20 jewels [1, 2]
Frequency: 21,600 vibrations per hour (3 Hz) [1, 2]
Power Reserve: Approximately 36 to 40 hours [1, 2]
Diameter: 26.00 mm (outer) / 25.60 mm (fitting) [1]
Thickness: ~4.50 mm [1]
Complications: Small second sub-dial at 6 o'clock; no date display [1, 2]

Pros and Cons

No Hacking: The second hand does not stop when you pull out the crown to set the time.
No Ghost Date: Because it lacks a calendar function, the single-position crown directly adjusts the time without any empty date-setting clicks.
Reliability: Part of the broader Sea-Gull ST17 family (derived from the ST16/Miyota-inspired


architecture), users and watchmakers generally regard it as a dependable, robust workhorse movement often found in affordable military-style or vintage field watches (such as Baltany and Militado)

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MIYOTA 8215

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The Miyota 8215 is a reliable, inexpensive, 21-jewel automatic watch movement manufactured in Japan by Citizen's Miyota division. [1, 2, 3]

Key Specifications

Type: Mechanical Automatic (with manual hand-winding)
Frequency: 21,600 vibrations per hour (3 Hz / 6 beats per second)
Power Reserve: Approximately 42 hours
Accuracy: -20 to +40 seconds per day
Diameter & Thickness: 26.00 mm diameter and 5.67 mm thick
Complications: Central hours, minutes, sweeping seconds, and a date window (typically at 3 o'clock) [1, 2, 3, 4]

Pros and Cons

Pros: Highly durable, easy to service, widely available for watch modding, and acts as a tough entry-level workhorse.
Cons: Features an indirect-drive second hand that can visually stutter or pause slightly when the watch is jolted. Older versions do not feature hacking (stopping the second hand to set the time), though modern iterations may include a stop-second device. Its uni-directional winding rotor can also spin freely and feel a bit loud or "rattly" on the wrist

This movement has been built into a (homage omega inspired seamaster 300 233) dive case.

100m water resistance.

Its OK for showering

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Very reliable.

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8205

The primary difference between the Miyota 8215 and the Miyota 8205 is their calendar complication: the Miyota 8215 features a date-only display, while the Miyota 8205 features a day-date display (showing both the day of the week and the date). [1, 2]

Both movements belong to Citizen’s rugged and affordable Miyota 8200 family and share the exact same core architecture, dimensions, and mounting specifications. [1, 2]

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Key Similarities

Frequency: 21,600 vibrations per hour (3 Hz / 6 beats per second). [1, 2]
Power Reserve: Approximately 42 hours. [1, 2]
Jewels: 21 jewels. [1, 2]
Winding: Automatic with uni-directional rotor winding and manual winding capability. [1, 2]
Accuracy Rating: -20 to +40 seconds per day. [1, 2]
Dimensions: 11.5 lignes (26 mm) in diameter and 5.67 mm in thickness. [1]
Hacking: Traditionally non-hacking (seconds hand does not stop when setting the time), though some modern revisions have featured hacking. [1, 2, 3]

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Key Differences

Feature	Miyota 8215	Miyota 8205
Complications	Date only (at the 3 o'clock or 6 o'clock position)	Day and Date display
Dial Layout	Cleaner, single calendar window	Wider or dual calendar windows for day/date
Case Compatibility	Fits date-only watch dials and cases	Requires a day-date dial aperture

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Note  the Miyota 8215 does not use a Pellaton winding system; it uses a traditional unidirectional winding rotor system. [1, 2]

• Winding Mechanism: The Miyota 8215 winds in only one direction (counter-clockwise/left rotation of the rotor as viewed from the back).
Pellaton System: The Pellaton winding system is a distinct, patented bi-directional pawl-lever system developed and used primarily by IWC (International Watch Co.), not Citizen/Miyota

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Disassembly Instructions

To disassemble a Miyota 8215 watch movement, let down the mainspring power, remove the hands, dial, and winding stem, and then systematically take off the automatic winding module, train bridge, wheels, and escapement.

Preliminary Steps

Remove Stem: Push the crown to position 0, press the small dimple on the setting lever with a tool, and pull out the winding stem.
Remove Hands & Dial: Use hand pullers to remove the hands, loosen the dial screws on the side of the movement, and lift off the dial.
Remove Canon Pinion: Pull off the canon pinion carefully with tweezers or hand pullers, watching out for the fragile second hand pinion.
Remove Date Mechanism: Unscrew the four screws holding the plastic calendar date wheel and lift it off. [1, 2, 3]

Disassembling the Movement Train & Escapement

Automatic Module: Turn the movement over and remove the oscillating weight and automatic winding bridge screws.
Train Bridge: Unscrew the three screws holding the train bridge and lift it off.
Gear Train: Remove the reduction wheel, ball winding wheel, ratchet wheel, fourth wheel, and escape wheel.
Barrel & Center Wheel: Take out the sweep second pinion, the mainspring barrel complete, intermediate wheels, crown wheel, and the center wheel bridge.
Pallet Fork: Unscrew and remove the pallet bridge and lift out the pallet fork.
Balance Assembly: Carefully remove the balance cock and balance wheel assembly last.

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