A thru-tubing centralizer 10, 102 positioned along a tubular T after passing through a small diameter tubing string TS centralizes the tubular within a large diameter casing string CS. The centralizer includes a body 12, 112 having a throughbore 14 for transmitting fluid through the centralizer, such that a drill motor M and rotatable bit B may be suspended from the tubular T below the centralizer. The centralizer includes a piston 20, 136 and arm support sleeve 30, 130 each movable axially between a set position and a released position in response to increased fluid pressure in the throughbore of the centralizer body. The compression spring 40, 156 biases an arm support sleeve to a set position, such that a plurality of circumferentially spaced upper arms 50, 116 and a corresponding plurality of circumferentially spaced lower arms 60, 126 are each inclined with respect to the centralizer body to maintain the centralizer in a set position for engaging the casing string. Increased pressure within the centralizer body moves the piston and the arm support sleeve to the released position, such that the upper and lower arms are retracted so that the centralizer may be retrieved to the surface through the tubing string.
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