The stem is movable within {TAIL ~ the annular protrusion} between at least a first position in which the orifice is closed when {HEAD ~ the valve} stem first end sealingly abuts the protrusion extension sealing edge and a second position in which the orifice is open, the valve stem first end is offset from the protrusion extension and the valve stem is restricted from moving away from the valve body by the groove rear wall 
The stem is movable within { TAIL ~ the annular protrusion } between at least a first position in which the orifice is closed when { HEAD ~ the valve } stem first end sealingly abuts the protrusion extension sealing edge and a second position in which the orifice is open , the valve stem first end is offset from the protrusion extension and the valve stem is restricted from moving away from the valve body by the groove rear wall
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