A bit of a joke but here is my contribution towards preventing the Bitcraze Crazyflie impacted bearing sindrome. A plastic washer with a height of 0.7mm and an internal diameter of 2.5mm and an outer diameter of less than 6mm. Preferably use plastic rather than metal washers because of weight.Print it and fit it between the Crazyflie motor bearing and prop. You can use an ordinary plastic washer with similar dimensions to the same purpose.