NPT, 1-9/16 in.. Take your sauna experience to a new level by adding a touch of your favorite fragrance with SteamSpa aroma Steamheads. Steamhead with Aromatherapy Reservoir in Matte Black diameterShips from manufacturer within 24 hours Enjoying a sauna infused with your favorite aroma for an incredibly soothing and calming effect making sure you’re at maximum relaxation.