The tracers are mounted on pistons which are received in cylinders and urged radially inwards by springs for retracting the tracers, the pistons being driven outwardly to displace the tracers into working positions abutting the well wall by {HEAD ~ supply} of actuating fluid to {TAIL ~ the cylinders} 
to
The tracers are mounted on pistons which are received in cylinders and urged radially inwards by springs for retracting the tracers , the pistons being driven outwardly to displace the tracers into working positions abutting the well wall by { HEAD ~ supply } of actuating fluid to { TAIL ~ the cylinders }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL