3 type B gas vent wall thimble from the RV collection this wall thimble is used for installation through a combustible wall to ensure clearance to combustibles is maintained. It also serves as a fire stop and can adjust from 4.125 to 7 to fit the thickness of the wall. The RV series offers round gas vent pipe and fittings for residential, commercial and industrial applications. It features a snap together, non-positional lock system, die formed ends and eliminates the need for all cutting, fitting and special engineering. It has an improved joint design provides for a rapid and secure connection in seconds. The use of our lock tab feature gives installers the option of engaging this tab instead of using sheet metal screws, when required by local code.