Preferably, the projection extends inwardly of {HEAD ~ the structure} and is of {TAIL ~ circular cross section} 
Preferably , the projection extends inwardly of { HEAD ~ the structure } and is of { TAIL ~ circular cross section }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL