Abstract: 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.