Metal-Fab 6MCHP 6" Style "M" B-Vent Vent Cap. 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 6MCHP 6" Style "M" B-Vent Vent Cap is ideal for preventing liquid from entering the vent system. The cap also breaks up the air and gas that leaves the system.