Abstract: A borehole underreamer, having an axial flow passage from end to end and useful with reverse circulation operations, has a cutter arm actuator which includes an axially movable tube. The tube carries an annular piston which moves in an annular piston chamber formed about the tube. The tube is driven, to open the underreamer, by air pressure supplied to the piston chamber via a double-walled drill pipe. An upper extension of the tube above the piston is removable from the remainder of the tube and defines reverse circulation air injection ports which are opened from the chamber to the axial flow passage when the underreamer is open but which are sealed from the chamber when the underreamer is closed. The extension may be replaced by a similar ported extension to adapt the underreamer to different reverse circulation conditions, or it may be replaced by a similar unported extension to adapt the underreamer for use in conventional circulation operations.