a method of rapidly assembling the rotor of an external - rotor motor onto a bearing support tube and in a predetermined axial position relative to the latter , includes the steps of : beginning at a rotor cup portion of the rotor , mounting on a rotor shaft : a compression spring , a retaining washer , and a plurality of rolling bearings ; then pressing into the bearing support tube , by means of a pressing - in force , the rotor , with the elements mounted thereon , the compression spring being compressed so that the rotor cup presses the washer into the tube ; subsequently removing the pressing - in force , and displacing the rotor shaft by means of the compression spring within the bearings so that the rotor assumes the desired predetermined axial position , relative to the tube .