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