which may be an aluminum casting an air gap is formed at 83 between the inner surface of the ring and the outer surface of the pulser rotor 31 and has a discrete discontinuity at the point at which the maximum diameter 53 decreases abruptly to the minimum diameter point 55 timing ring 71 includes magnetic pole pieces four such l shaped pole pieces numbered 85 87 89 and 91 being shown in the embodiment of fig2 and 3 the pole pieces 85 87 89 and 91 are embedded in spaced relation in associated recesses formed in the inner surface of timing ring 71 a portion of the l shaped pole piece extends substantially vertically see fig2 and 3 and the other portion of the l shaped extends substantially horizontally on the periphery of the timing ring 71 a retainer ring 78 which is of resilient material or which can include resilient fingers a portion of the ring only being shown in fig4 is affixed over timing ring 71 the ring 78 or portion thereof press against timing ring 71 to prevent vibration of the timing ring and to generally hold the timing ring 71 in the position to which it is adjusted the configuration of the rotor assembly 31 provides the advantage that if the rotor assembly 31 is rotated in a first or clockwise direction in fig3.