A torque bias coil spring for a camshaft phaser disposed in an annular well formed in the phaser rotor and having a central arbor for supporting the spring. The radius of the central arbor is such that the spring has radial operating clearance of 5-10%, as in the prior art. However, the improved arbor includes regions of higher radius at the arbor ends supporting the innermost and outermost coils of the spring. These regions serve to prevent the spring from being laterally displaced during torsional motion, thus minimizing the relative motion at the spring contact points, reducing stress on the spring, and improving the efficiency of the spring by reducing frictional hysteresis.