Selkirk 3 in. Dia. Galvanized Steel Wall Thimble (Pack of 2)Selkirk VP pellet pipe thick wall thimble is used to provide the required 3 inch clearance to combustible materials. Thimble expands from 6 1 2 inch to 11 inch thick. Features proven gasketed coupling joints that connect in seconds to assure joint integrity. Available in 3 inch and 4 inch diameters of 304 stainless steel inner wall and paintable galvanized outer wall. Internal spacing bead assures uniform insulating air space between pipe walls and