The air gap should be set wide enough to prevent striking of the armature and coil faces under normal operation.  At the same time, the air gap should be set close enough so the feeder bowl will not exceed its current limitations as stamped on the name plate.  Note:  Too close will cause striking, too much gap will heat up the coil and lower maximum speed of the feeder bowl.  The rule is as close as possible without striking.

To adjust the air gap:  use a set of ignition feeler gauges to ensure consistent spacing across the coil face (for the bowl, .030" for non-rectified or .060" for rectified;  for the hopper, .045" for the in-line, .050").  Never open the air gap more than necessary as an excessive air gap draws more current and reduces the power of the electromagnet resulting in poor performance.