A camshaft phaser for controllably varying the phase relationship between a crankshaft and a camshaft of an internal combustion engine includes a housing connectable to the crankshaft and having a housing bore extending therethrough along an axis. A back cover is attached to one axial end of the housing while a front cover is attached to the other axial end of the housing. An output hub connectable to the camshaft is disposed coaxially within the housing and captured axially between the back cover and the front cover. A harmonic gear drive unit is disposed operationally between the housing and the output hub and is connected to a rotational actuator for imparting rotation on the harmonic gear drive unit such that rotation of the harmonic gear drive unit by the rotational actuator causes relative rotation between the housing and the output hub.