Abstract: An adjustable drop-in concrete anchor includes a substantially cylindrical tubular base, the base having a lower end portion having a plurality of longitudinally extending slots defining a plurality of longitudinally extending expansion walls having a wall thickness greater than a wall thickness of the base above the lower end portion, and having an internally threaded upper end portion. The anchor includes an expander plug having at least portion having an outside diameter slightly less than an inside diameter of the base above the lower end portion and greater than an inside diameter of the base at the lower end portion. An adjustment insert disk has external threads extending around at least part of an outer circumferential surface thereof, the external threads mating with the internally threaded upper end portion of the base, and having a slot extending from a portion of the circumferential surface to at least a central axis thereof.