Glass reducer downstems in 10, 12, 13, 14, 15, 17 & 25cm lengths (measured from the top of the grind (not the very top of the stem) to the bottom of the stem).  Available with and without diffuser slits (diffuser/straight cut).  The bottom of the stem does not taper and finishes with a opening the same size as the OD of the stem.  Your ash will not collect at the bottom of this stem like it can on some cheaper glass stems.  

Inner diameter of stem approximately 11mm.  Outside diameter of stem approximately 15mm.