BSELKIRK CORP 187704 Wall Thimble, 4-Inch ReviewbP PP SELKIRK CORP 187704 Wall Thimble, 4-Inch P4" 4RV-WT Wall ThimblePAdjusts 4-18" to 7" to fit wall thicknessPRound Type B gas ventPWhen installed according to manufacturer specifications, the wall thimble provides the required 1 inch air space to combustiblesPTested and listed per UL-441 for installation as type "B" and B-W gas vents to be installed in single and multi-story buildingsP