conduit 13 the upward or downward movement of the pin 9 adjusts the axial position of the stop face 26 within the conduit 13 thereby adjusting the limit of movement for the upper end 23 of the return plunger 20 as the underreamer 10 is moved to its expanded position when upward movement of the plunger 20 is limited to a greater extent by downward movement of the pin 9 within the conduit 13 the distance to which the cutter assemblies 40 are able to be radially extended is correspondingly limited conversely upward movement of the pin 9 within the conduit 13 allows the cutter assemblies 40 to move radially outwardly to a greater extent adjustment of the location for pin 9 in this manner thus results in the diameter of the tool with the cutter assemblies 40 to be adjusted to conform to a desired diameter precise adjustment of the movement of the cutter assemblies 40 is accomplished by placing a ring gage of a type known in the art for the measurement of tool diameter across the diameter of the cutter assemblies 40 when the cutter assemblies 40 are placed in a radially expanded position as is illustrated in fig6 the axial location of the pin 9 is then adjusted until the diameter of the expanded cutters essentially matches that of a desired hole opening diameter as measured by the ring gage in this manner an underreamer constructed in accordance with the present invention is capable of being easily adjusted to expand to a variety of different radial sizes without.