An assembly for a coupling together rotatable components according to an example of the present disclosure includes, among other things, a rotor shaft extending along a centerline axis, the rotor shaft defining an inner bore extending from an axial end of the rotor shaft, a gear mounted on an outer diameter of the rotor shaft, a lock nut threadedly engaged with the outer diameter of the rotor shaft such that the lock nut abuts against an axial end face of the gear, and a retainer including a step portion extending from a head portion and including a projection portion extending from the step portion. The projection portion is insertable into the inner bore to bias the lock nut against the axial end face of the gear, and the step portion is dimensioned to establish an interference fit with the rotor shaft along the inner bore. A starter assembly and a method of assembly are also disclosed.