It also eliminates {HEAD ~ the necessity} for exactly positioning {TAIL ~ the closed valve disc} to effect a leakproof closure 
for exactly positioning
It also eliminates { HEAD ~ the necessity } for exactly positioning { TAIL ~ the closed valve disc } to effect a leakproof closure
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH