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 {HEAD ~ working positions} abutting {TAIL ~ the well wall} by supply of actuating fluid to the cylinders 
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 { HEAD ~ working positions } abutting { TAIL ~ the well wall } by supply of actuating fluid to the cylinders
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