Abstract: The invention relates to an applicator device for applying, e.g., a viscous fluid or a gel, with the use of a self-tapping piston. The device comprises a piston provided with a blade, the cutting edge of which is inclined in relation to the longitudinal axis of a rod passing through the piston. The cutting edge of the blade projects into a space defined in the piston through which the rod passes and bites into the relatively soft, smooth outer surface of the rod as the rod is rotated. The blade cuts into the surface of the rod during such rotation in a substantial spiral direction, causing the piston to move along the rod.