The machine has been constructed for automatic assembly of the “box” gear unit on the bottom bracket. The operation is performed as follows: in the first station the frame in upside–down position is centered on the bottom bracket by an alignment pin and then pneumatically clamped. The table revolves and transfers the frame to the next assembly station where the two parts to be assembled have already been drawn from the feeder and prealigned. The screwing groups mounted on movable slides insert the pieces in the bottom bracket, prescrew them and finally lock them with the preset torque. At the end, the frame rotates to station three from which it is picked up. The loading, cap mounting and unloading operations occur simultaneously.