Fits neck: 21-25mm. For use with Buchner funnels and filter flasks, these provide an easier alternative to boring a large hole in a rubber bung. The rubber cone provides a seal between the filter flask and the Buchner Funnel. The seal is not formed at the stem of the funnel, but at the tapered base, when the funnel is lightly pushed down into the cone, which sits in the neck of a filter flask. Choose the rubber cone size which fits part way into your flask neck e.g. a 23mm ID flask neck requires a rubber cone with 19mm bottom dia. & 27mm top dia.