Metal-Fab 4" x 18" Type B Double Wall Round Gas Vent Pipe UL Listed with SureLock Technology (#4M18) PLEASE NOTE: Vent overlaps 1.5" when joined together, thus assembled length is 1.5" less than listed length. SURELOCK B-VENT LOCKING SYSTEM: The design of the SureLock B-Vent System is unique. The male end of the pipe features a square dimple, positioned in the exact location to match the tab that is located on the female end of the pipe. When the pipe is joined and twisted until it stops, the SureLock tab can be pushed into the square dimple, therefore creating a positive twist lock that cannot be twisted apart. So unique that a patent is pending.