This series of {HEAD ~ the actions} are repeated to provide {TAIL ~ a continual reciprocal movement} of the piston 
This series of { HEAD ~ the actions } are repeated to provide { TAIL ~ a continual reciprocal movement } of the piston
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH