Abstract: An adjustable quick change chuck jaw apparatus for attachment to a master jaw in the front face of a rotary chuck allows a machinist to quickly change the chuck jaws from one diameter to another. A generally rectangular jaw body is connected to the master jaw of a rotary chuck and extends radially outward from the central rotational axis of the chuck. A jaw member is slidably mounted on one side of the jaw body to travel longitudinally relative thereto and has a workpiece gripping portion extending perpendicularly outward. A rotatable shaft mounted in the jaw body has a threaded segment configured to become engaged with mating threads on the jaw member when rotated to an engaged position and to become disengaged therefrom when rotated to a disengaged position. A spring biased locking pin in the jaw body allows selective engagement with the shaft to prevent rotation thereof in a locked position and to become disengaged therefrom to allow rotation in an unlocked position.