Selkirk 187704 Round Type B Gas Vent Wall Thimble, 4", #4RV-WTThe RV line of Selkirk round vent pipe and fittings is designed for use on approved gas appliances in residential and light commercial and industrial applications. Round vents for large appliances will be found in the QC section which follows. Size 4" Part No. 4RV WT Measures: 4 58" x 8 1 4" Used to ensure clearance to combustibles is maintained Wall Thimble also serves as a firestop Adjusts from 4 1 8" to 7" to fit thickness of wall