The outer diameter of the body 44 adjacent the top end 44A and including the portion of the body 44 having the center portion 46, has an outer diameter smaller than the outer diameter of the remainder of the body 44 adjacent the bottom end 44B. This allows the bottom end 44B to be mounted in {HEAD ~ the vent pipe} 40 without the possibility of {TAIL ~ the body} 44 being inserted into the vent pipe 40 such a distance as to cover the air inlets 54  
The outer diameter of the body 44 adjacent the top end 44A and including the portion of the body 44 having the center portion 46 , has an outer diameter smaller than the outer diameter of the remainder of the body 44 adjacent the bottom end 44B. This allows the bottom end 44B to be mounted in { HEAD ~ the vent pipe } 40 without the possibility of { TAIL ~ the body } 44 being inserted into the vent pipe 40 such a distance as to cover the air inlets 54
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