A shaft assembly for a powertrain includes a shaft having a cavity extending at least partially from a first axial end to a second axial end of the shaft and opening at at least one of the first axial end and the second axial end. For example, the shaft may be a balance shaft, a camshaft, or a transmission shaft. A first core plug is disposed in the cavity. The shaft and the core plug may be the same material, or may be different materials. The shaft may have a first density, first cross-sectional area, or first area modulus, and the core plug may have a different second density, second cross sectional area, or second area modulus which may be less than the first density, the first cross-sectional area, or the first area modulus.