Abstract: An apparatus for measuring a well bore wall comprises a casing connectable in line with a tool string having a central passage therethrough and extending between first and second ends and a plurality of longitudinally extending biasing elements extending longitudinally along the casing between first and second ends wherein each of the second end of the biasing elements is connected to the casing. The apparatus further comprises a sensor located along a midpoint of each of the biasing elements and an engagement body located within the central passage of the casing longitudinally displaceable therein between first and second positions, wherein the engagement body is connected to the first end of each of the biasing elements such that displacement of the engagement body within the central passage from the first to the second positions compresses and radially extends the biasing elements so as to engage the sensors against the well bore wall.