3" Type B Gas Vent Wall Thimble 3GVWT This wall thimble is used for through-the-wall system to provide clearances for vent from combustibles. Adjusts from 3 ½" to 6" to accommodate various wall thicknesses. For Category 1 gas appliances 1" clearance to combustibles Easy-fit twist-lock design with no fitting hassles 0.012" aluminum inner 0.018" galvanized outer