SXK Insider RBA for Billet Box Rev4 Boro Tank Easy access air flow control, dual uprights design and huge channels allow for easy regeneration, quick absorption and large air flow for coils, even at higher wattage. Lower airflow design for more aromatic steam production Included in the kit are 2 chimneys: one wide barrel (4mm) for great steam production and a narrower chimney (3.2mm) for aromaticity.