Abstract: A well string injector has a frame, with a guide at one end for guiding well string along the frame and a support at the other end for directing the well string towards a target. At least one of a pair of longitudinally spaced clamps is movable with respect to the other and at least one clamp is oriented on the frame to provide clamping forces on well string by relative movement between clamping members, each carrying a well string gripping die, predominantly in a transverse direction perpendicular to the frame longitudinal direction. A third well string gripping die may be carried by the clamp for location between parallel spaced well string clamped by the clamp. A well string gripping die is provided that includes a gripping surface that induces an optimized stress distribution in well string clamped by the gripping die when the well string is in tension. A well string gripping die is provided that includes one and preferably a pair of barrier walls for preventing walk out of well string.