Abstract: Employee # 1, working in the hydrogen production unit of a petroleum refinery, was using a Hytorc 4XCLT hydraulic wrench to remove nuts from studs that were holding the head onto the end of a heat-exchanger. The wrench is designed to be abutted against a stationary object and held in place during operation, until the nut breaks free. The employee placed the wrench on the nut with his left hand and activated the wrench using a remote button with his right hand. He had not positioned the wrench against a stationary object, and his fingers were between the wrench handle and the heat-exchanger flange. As the wrench tightened on the nut, it turned and pinched the employee's hand against the flange. Employee # 1 suffered a partial amputation of his left ring finger.