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