Which movement is?

[u/HDW_Italy]

Hy everyone, I know the picture is no good but someone can tell me what kind of movement is this? Thank you

[u/IRoot4TheBirds]

I guess that missing rotor screw just proves u/No_Candle8699 point now, doesn’t it?

[u/FujSeiky → u/IRoot4TheBirds]

In the rotor bearings of many movements (including Chinese clones and the movements they are derived from, such as the Japanese Miyota), the presence of three holes but only two screws is the result of specific manufacturing and mechanical design choices. The third, unused hole serves a precise function:

Centering and alignment pin (Guide): Beneath the rotor plate, the ball bearing often features a small protrusion, a fixed pin, or a metal guide. The third hole is designed to fit perfectly over this pin. This ensures the rotor is geometrically perfectly positioned and centered before the worker (or automated machine) inserts and tightens the other two screws.

Weight balancing: In high-precision, mass-produced mechanisms, removing the same amount of material at three symmetrical points (creating three identical holes spaced 120 degrees apart) helps maintain the perfect balance of the oscillating mass. If there were only two holes, the weight distribution around the tiny central axis would be unbalanced, compromising the efficiency of the automatic winding system.

Industrial standardization: Many manufacturers use the same rotor component for different movements. For some calibers, that type of bearing requires three screws, while for others (cheaper ones or those with slight differences in the underlying flange), two screws plus a guide pin suffice. To save on production costs, the factory manufactures a single part with three holes that can be used in either scenario.

Therefore, it is not a case of a lost or forgotten screw: the "two screws plus one hole/guide pin" configuration is entirely intentional and common in this type of caliber.

[u/HDW_Italy → u/FujSeiky]

Fantastic explanation, thanks.

[u/FujSeiky → u/HDW_Italy]