Fountain Mini Piston Converter for Kaweco Sport Fountain Pens. NOT the Liliput. Mini piston converter for Kaweco Sport fountain pens. Approximately 1-5/16" (33 mm) long closed and 2-1/16" long (52 mm) with the piston fully extended. 7 mm diameter - slightly smaller than a standard international ink cartridge when closed and slightly longer when the piston is extended. 7 mm diameter. NOTE: the piston can not be fully extended inside an AL sport pen.