[nberga]

What material is the pinion made of? If plastic, then it might have split and that is why it is not gripping the shaft.

There are generally two diameters of motor shafts, 1.5mm and 2mm. I've just measured my Slot.it motor shaft and it is 2mm.

The Pinion on the Rear Axle is plastic but is working fine, the problem is the new motor, does not hold that in place.
With the original motor the shaft of the motor would go further and locked the rear axle so it does not move and work, with the new motor the V12/4 from SLot.it the shaft os this motor does not protrude on to the rear axle and it comes loose and off the motor pinion.