Abstract: An apparatus for cross-sectionally enlarging a chimney flue includes a fluid motor; a milling cutter mounted on the fluid motor and being driven thereby for removing constructional wall material from walls defining the flue; a suspension device for supporting the fluid motor and the milling cutter as a unit and for raising and lowering the fluid motor and the milling cutter in the chimney flue; a fluid pressure source; a hose connecting the fluid pressure source with the fluid motor for supplying the fluid motor with pressurized fluid from the fluid pressure source; a guiding device arranged about and travelling with the fluid motor for engaging the flue walls to guide the fluid motor in the flue; and a device for varying a radial position of the guiding device relative to the motor axis.