This is a 3/8” diameter, three-ply rope made from Fiberfrax wicking, which makes a great seal between the burner and the boiler. Simply lay a length of this soft, resilient rope on the edge of your burner pan between the pan and the liner before securing the burner to the boiler. Sold by the foot. 3’ for 14” burner fitting inside the boiler ring, 4’ for 16”, 5’ for 18”, 6’ for 20” & 23”, 7’ for 26”. Add 1’ if your pan is fitted outside of the boiler.