An expandable shaft assembly including an axially extending shaft with an axial axis, an extending shaft with an axial axis, an external dimension, at least one shaft end and a bore extending generally axially from the at least one shaft end. The shaft assembly also includes a second element, as an axle cap, with an axially extending, externally threaded portion with a first thread flank, a shoulder portion extending in a generally radially outwardly direction. A radially extending end face is also provided that is axially constrained relative to the shaft, wherein the radially extending end face is one of (1) integral with the shaft, and (2) on a separate element. The bore of the shaft includes an axially extending, internally threaded portion with a second thread flank, wherein at least one of the first and second thread flanks are inclined. The externally threaded portion of the second element is threadably engaged with the internally threaded portion of the shaft such that the first thread flank contacts the second thread flank. In addition, the shoulder portion of the second element is pressed against the end face and the inclined thread flank is wedged against at least one of the first thread flank and second thread flank to provide radial expansion of the external dimension in an expansion region.