More preferably, {TAIL ~ the duct} is defined by {HEAD ~ a groove} in an outer surface of the hub 
More preferably , { TAIL ~ the duct } is defined by { HEAD ~ a groove } in an outer surface of the hub
OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH