Abstract: The invention herein relates to an undercutting device which is provided with a substantially tubular body adapted to be placed within a parallel-sided hole. A rotary percussion head is provided at one end of the body and has one or more cutting tools which extend radially therefrom at an obtuse angle and which cutting tools are adapted to cut outwardly from the radius of the body, means are provided for rotating the head about the axis of the body and for operating the percussion head at one predetermined depth in the hole so that the cutting tools may act on the wall of the parallel-sided hole.