Abstract: Apparatus for modifying building blocks which vary somewhat in size to provide spaced-apart depressions in the top and bottom surfaces thereof with the bottoms of the depression in the bottom surface being at a uniform distance from the top surface of the blocks comprises groove cutting and associated block drilling means. The drilling includes movable plates carrying a plurality of drills, means to move one of the plates before the other, and means to position the blocks for proper drilling. Also, a plurality of cutters are used to speed the cutting operation.