A close-up of the work end of the motor, showing the round dents in the casing. Note also the recess into which the metal caps click and lock into place. While in this state, apply a drop of thin oil to the brass bushes where the spindle leaves the casing. The motor should spin rapidly and freely (apart from the slight lumpy feeling caused by the magnets). If it feels rough, it may be that some dirt has got between the casing and the armature. You may see scratch marks on the armature. Spin the motor and blow hard into the motor windings as it spins. Do this a few times to try to clear any debris.