Abstract: The apparatus comprises means for replaceable coupling thereof to a standard raise boring head for extending the effective cutting radius of the head. The apparatus comprises a raise boring head radius extension element which is pinned to struts extending from a circular hub; the hub, in turn, envelops a terminal end of the stem of the raise boring head. Additionally, pin-holed clevises borne by the extension elements mate with like pin-holed lugs which extend, radially, from the principal cutter bracket mounting platform of the standard raise boring head. Expansion pins are engaged with the lugs and clevises, and also secure the struts both to the hub and to the extension element, to simplify assembly and disassembly. In typical use, a plurality of radius extension elements are coupled to the standard raise boring head, the elements being arrayed circumferentially thereabout.