SELKIRK CORP 187703 3-Inch  Wall Thimble 3", 3RV-WT, Wall Thimble Adjusts 4-1/8" to 7" to fit wall thickness Round Type B gas vent When installed according to manufacturer specifications, the wall thimble provides the required 1 inch air space to combustibles Tested and listed per UL-441 for installation as type "B" and B-W gas vents to be installed in single and multi-story buildings