A vane-type camshaft phaser for varying the timing of combustion valves in an internal combustion engine includes a seat formed in the sprocket at the appropriate position of intermediate rotation and a locking pin slidably disposed in a vane of the rotor for engaging the seat to lock the rotor at the intermediate position. A bias spring system disposed on a cover plate urges the rotor toward the locking position from any position retarded of the locking position. A first spring system embodiment comprises a pair of compression spring assemblies. A second spring system embodiment comprises an internal torsion spring. In each embodiment, the phaser may be assembled without having the spring system coupled to the rotor, thereby overcoming a rotor cocking problem inherent in prior art phasers, assuring reliable mounting of an assembled phaser onto an engine camshaft.