Diffuser Downstem. These downstems have cuts at their ends to diffuse the smoke as it enters your bong. The clear glass with colored logo gives it a clean look. Diffuser Downstem. These downstems have cuts at their ends to diffuse the smoke as it enters your bong. The clear glass with colored logo gives it a clean look. Joint Size: 19mm Male Joint / 14mm Female Joint Lengths: 4" 10cm / 4.75" 12cm / 5.5" 14cm / 6.25" 16cm