Answer first
The Royal Oak 5402ST runs on Audemars Piguet Calibre 2121. That movement is a thin automatic that Jaeger-LeCoultre built as the Calibre 920. No replica factory makes a true copy at scale. A watch sold as a 1:1 replica of this reference almost always carries a Miyota 9015, an ETA 2892-A2, or a Chinese thin automatic with added plates and rotor engraving. The case is cut to fit that movement, so thickness and hand height differ from the original.
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What the original movement is
Calibre 2121: automatic, 36 jewels, 19,800 vibrations per hour (2.75 Hz), about 40 hours of power reserve, date at 3 o'clock, no seconds hand. The movement is about 3 mm thick. Jaeger-LeCoultre designed the base as the 920 and supplied it to Audemars Piguet, Patek Philippe, and Vacheron Constantin. Production numbers for the 920 family were small.
ap royal oak 5402st rep vs gen dimensions
Movements used in replica 5402ST watches
Miyota 9015
Japanese automatic. 24 jewels, 28,800 vph, about 42 hours of reserve, 3.9 mm thick, date at 3 o'clock. The date position fits the 5402ST dial. The beat rate does not. It is the most common base in mid-price replicas.
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ETA 2892-A2
Swiss automatic. 21 jewels, 28,800 vph, about 42 hours of reserve, 3.6 mm thick, date at 3 o'clock. Thin enough for slim cases. Cost is higher than Miyota, so fewer replica listings use it. A Swiss movement claim in a listing points here or to a 2824.
Chinese thin automatic
Several factories sell automatic movements with a decorative bridge plate and an engraved rotor that copies the 2121 layout. Jewel count and finish vary. Some run at 21,600 vph, some at 28,800 vph. The plate is cosmetic. The real gear train sits below it.
Real 2121 or JLC 920
Loose 2121 units and JLC 920 units appear at auction and from dealers. Prices run into four figures in US dollars. A replica case will not accept them without new movement rings, and the stem height may not line up.
How to check a movement claim
- Beat rate. Use a timegrapher app or a watchmaker. A genuine 2121 reads 19,800 vph. A 9015 or 2892 reads 28,800 vph.
- Rotor. Look at engraving depth and font. Cosmetic plates show gaps at the edges.
- Jewel count. 2121 has 36. 9015 has 24. 2892-A2 has 21.
- Seconds hand. The 5402ST has none. A running seconds hand means the movement is not a 2121.
- Case thickness. Measure with calipers. A 9015 build adds about 1 mm over the original.
What 1:1 covers
The term describes the outside. Bezel screws, dial texture, bracelet taper, and case finishing. It does not describe the movement. Ask for a photo of the movement before payment. Ask for the beat rate. A seller who cannot supply either is selling an unknown caliber.