A variable camshaft timing phaser (10; 110; 210) in which a rotor (12; 112;212) that is secured to a rotatable camshaft is selectively advanced or retarded in position relative to a surrounding rotatable housing (14; 114, 214), the rotor having at least one outwardly extending vane (20; 120; 220) that is received in an inwardly facing recess (18; 118; 218). Pressurized oil is selectively delivered to one of an advance portion or a retard portion of the recess, and simultaneously withdrawn from the other of the advance portion and the retard portion, by adjusting the axial position of an axially shiftable spool valve (22; 122; 222). The spool valve has a null position, and the relative positions of the rotor and the housing are positively locked in position when the spool valve is in its null position by a locking pin (48; 148; 248). The locking pin is resiliently biased towards a locking position by a spring (52; 152; 252), and is urged away from its locking position by pressurized oil from a source.