Abstract: Disclosed is an optical encoder structure which is easily attached to a motor case and shaft in either the factory or field, and which automatically achieves, by its construction, both axial and radial alignment. This is achieved by first securing to a housing by a novel latch, a mounting plate which is spaced a predetermined distance from the housing. A rotatable encoder disc, having a conically shaped hub is placed in an aperture in the housing, the conically shaped hub serving to self center the disc radially relative to the housing. The assembly is pushed onto the motor shaft, and the disc is secured to the shaft. The plate is then attached to the motor case and the housing rotated. This moves the housing into an abutting relationship with the plate and spaces the housing a predetermined distance from the disc. Thus the disc is both axially and radially positioned, no instruments being required to accurately position the housing with respect to the disc or motor.