4" 4RV-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. 4" 4RV-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.