Aluminum has become a widely used material in the automotive industry due to the need for production of lighter automobiles. As the number and complexity of aluminum products increase, it becomes necessary to improve methods used to join these parts. Thermal interference fitting is one of these methods. It is not possible to calculate interference fit stresses accurately for all conditions using traditional calculation methods, and a method to calculate separation frequency of interference fitted aluminum parts is not available. This study investigates elastic-plastic interference fit stresses and separation frequencies of lightweight metal gear and steel shaft interference fitted joints (which have begun to be used widely in designs) using analytical and numerical calculation methods. It was found as a result of the study that plastic deformation occurred due to decreasing diameter, and increasing interference and numerical calculation methods produced accurate results. It was determined as a result of numerical analysis that the separation frequency of lightweight metal gears was lower in smaller interference fit diameters compared with steel gears, and that this increased in connection with increasing interference fit diameter and was found to be higher compared with steel gears.