Polini secondary gearing increases the overall gear ratio and requires more power from the engine in order to take full advantage of it. The gearing is delivered including a spacer that ensures that the shaft is pushed in through the gear correctly, as well as another piece to ensure that the shaft is not pushed out when the small gear is pressed on.
If necessary, see in the video below how to push a leverage.
Note: The gear unit replaces the two original gears, these must therefore be pressed off and the Polini gears pressed on.