A wrench head comprises a working axis, a first jaw, a second jaw, and a third jaw. The first jaw comprises first-jaw arcuate convex contact surfaces. The second jaw is coupled with and pivotable relative to the first jaw and comprises a second-jaw arcuate convex contact surface and a second-jaw planar contact surface. The third jaw is coupled with and pivotable relative to the second jaw and comprises a third-jaw arcuate convex contact surface and a third-jaw planar contact surface.