Abstract: A prime mover includes a shaft assembly that is supported for reciprocal movement between first and second ends of a stroke, and that includes first and second portions having a first material, and a third portion in between the first and second portions, having a second material, different from the first material. A single inductive proximity sensor makes a first indication when proximate the first material, and makes a second indication, when proximate the second material. A portion of the sensor is proximate the first portion when the shaft is at the first end of the stroke, the same portion of the same sensor is proximate the second portion when the shaft is at the second end of the stroke, and the same portion of the same sensor is proximate the third portion the entire time the shaft is in between the first and second ends of the stroke.