Metal-Fab 4MVC 4" Style M B-Vent Vent Collar. 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 4MVC 4" Style M B-Vent Vent Collar features a patented locking system ideal for a secure lock. The Tab that is included allows you to push-to-lock and pull-to-disassemble. This Vent Collar, along with the easy connections, is ideal for quick installation and long lasting performance.