More preferably, the walls of {HEAD ~ the groove} converge to a point to define {TAIL ~ a circumferential valve seat} 
More preferably , the walls of { HEAD ~ the groove } converge to a point to define { TAIL ~ a circumferential valve seat }
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL