Metal-Fab 4M18 4" Style M B-Vent 18" Length. These light commercial and residential B-Vents feature a positive twist locking system designed for easy assembly and dis-assembly without bending or harming the product. The B-Vent Locking System comes standard on 3"- 6" pipes. The Metal-Fab 4M18 4" Style M B-Vent 18" Length also features a patented locking system ideal for a secure lock. This shorter run is ideal for getting the air vented and making runs without cutting and ruining the pipe.