Use this thimble to pass through a combustible wall and into a masonry chimney. Sold in two parts, the thimble cover assembly and the thimble itself, both are required. You need one set of thimble cover per installation. The thimble cover assembly protrudes 13" into the room. You can use one or two thimbles, depending on the thickness of the combustible wall. A single thimble, 8 inches long, will pass through a 7-inch wall. It can go through a wall up to 15 inches thick by cementing two thimbles together with stove cement and gasket. Thimbles can be cut shorter with a hacksaw to achieve desired length. Thimble requires a 15" or 17" square opening. for use with 6" or 8" diameter tubing, respectively. Thimble cover mounting plate is 17 x 17 inches for the 6-inch thimble and 19 x 19 inches for the 8-inch thimble. : Type: Insul-Flue Deck Assembly Only Specifications: Overall Dimensions: 6" (W).