A camshaft phaser includes a housing with an array of internal splines formed within a bore. A harmonic gear drive unit is disposed within the housing and includes a circular spline and a dynamic spline, a flexspline disposed radially within the circular spline and the dynamic spline, a wave generator disposed radially within the flexspline, and a rotational actuator connectable to the wave generator. One of the circular spline and the dynamic spline is fixed to the housing. A hub is rotatably disposed radially within the housing and attachable to the camshaft and fixed to the other of the circular spline and the dynamic spline. A back plate has an array of external splines engaged in a sliding fit with the array of internal splines for transmitting torque from the back plate to said housing. The back plate also has an input sprocket for receiving rotational motion, in use.