A method of biasing a worm shaft of a motor assembly including the steps of: providing a motor assembly having a housing defining an inner chamber, an access aperture spaced therefrom, a shaft aperture in communication with both the chamber and access aperture, and a shaft rotatably supported by and at least partially disposed in the shaft aperture; providing an ultrasonic device having a horn for producing vibrations; providing a slug having a body defining a first shape for being disposed in the access aperture, transformable to a second shape whereby the slug melts in response to predetermined vibrations; placing the slug in the access aperture; actuating the ultrasonic device so as to transform the slug to the second shape such that the slug melts and at least partially flows into the shaft aperture to abut the terminal portion.