A motor mounting device includes a polymeric motor housing having a cylindrical housing section for receiving motor components including a rotor. The housing section incorporates a detent feature disposed along the interior surface, with the interior surface inclined to gradually converge in a distal direction. The housing further has a distal end section that includes a first bearing seat centered on the housing axis. A rotor retaining member has an annular outer surface terminating at an arcuate peripheral edge, and comprises a second bearing seat. The rotor retaining member is insertable distally into the cylindrical housing section, following insertion of a rotor, to engage the first and second bearing seats respectively with distal and proximal bearings on a rotor shaft, to secure the rotor for rotation relative to the housing. During such insertion, the retaining member outer surface and the interior surface of the cylindrical housing section interact to center the retaining member.