Anti-ovaling mechanical coupler

Disclosed is a coupler that grips the internal surface of a torque receptor such as a pipe or driveshaft to be turned. When torque is applied to the coupler, one or more gripping surfaces are forced apart by a cam mechanism. The gripping surfaces tightly engage the internal surface of the pipe in response to movement of an internal cam. Torque applied to the wrench causes the pipe to turn. Because the wrench grips the internal diameter of a torque receptor, it can be used without damaging the external surface of the torque receptor, and used in very confined spaces. The gripping surfaces distribute the gripping load evenly across the pipe, which minimizes any deformation of the pipe. The pipe is engaged by the gripping surfaces beyond the threaded portion of the pipe, where the pipe has more structure, to prevent ovaling.