A camshaft phaser includes a housing with a harmonic gear drive unit disposed therein. The harmonic gear drive unit includes a circular spline and a dynamic spline, a flexspline disposed within the circular spline and the dynamic spline, a wave generator disposed within the flexspline, and a rotational actuator connectable to the wave generator. A coupling adapter is disposed coaxially within the housing bore and fixed to the wave generator and supported in the housing by a bearing which is press fit onto a bearing surface of the coupling adapter. The coupling adapter has a coupling adapter bore with opposing drive lugs extending radially inward therefrom which are axially coincident with the bearing surface. A coupling of the rotational actuator is disposed within the coupling adapter bore and has opposing drive slots which receive the opposing drive lugs, thereby transmitting motion from the coupling to the coupling adapter.