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