Abstract: An internally mounted, portable clamping device for preventing distortion during welding of branch fittings on thin walled pipe. The clamping device comprises a pair of generally arc-shaped clamp halves which are inserted into a pipe and engaged by a screw inserted through a branch fitting which has been tack-welded to the outer surface of the pipe. Turning the screw in one direction moves the clamp halves radially apart until they engage and apply a radially tensive force on portions of the inner surface of the pipe immediately adjacent and opposite the fitting, thereby preventing the pipe from distorting during finish-welding of the fitting. The clamp is disengaged by turning the screw in the opposite direction.