Apparatus for selectively breaking out deposits from and/or lining of rotary kilns, which comprises means for moving the apparatus lengthwise in a kiln, a substantially conical front end having a mounting fixture for attaching the conical front end to the rest of the apparatus, a substantially rounded rear end, a plurality of outwardly curved longitudinal ribs having an outermost point, said ribs being spaced apart from each other around a notional centerline, a transverse rib for securing said spaced apart positioning of said longitudinal ribs, said longitudinal ribs connecting said front and rear ends, a working disk having working elements thereon, said working disk being located substantially between said longitudinal ribs and said rear end, an outer notional straight conicity line drawn between the outer periphery of said working disk and said outermost point, and a momentary inner notional straight conicity line drawn between a point on said working disk which point is within the outermost diameter of said disk by a distance to which working elements have momentarily penetrated into the material to be broken out and a momentary tangential contact point on the outwardly curved longitudinal ribs, the position of said momentary contact point being determined by the extent of the momentary penetration into the material to be broken out, said inner notional conicity line being formed inwardly of said outer notional straight line during operation of the apparatus, and said outer and said inner notional straight conicity lines coincide with each other when the apparatus is not engaged in breaking out operation.