The vent hole is closed by {HEAD ~ the base plate} of the fixed scroll member for {TAIL ~ the majority} of its operational cycle 
for
The vent hole is closed by { HEAD ~ the base plate } of the fixed scroll member for { TAIL ~ the majority } of its operational cycle
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH