Features: To prevent water backflow to damage the front end of the pump or other products and equipment. After the addition of the stop valve, the pump can be placed above or below the surface of the water to prevent the risk of water backflow when the pump is shut off. Each outlet needs to connect a stop valve, two-headed pump needs two stop valves, and so on. Description: Quantity: 5pcs/bag Overall length: 36mm / 1.42in, the diameter of the middle circle: 8mm/0.31in Two ends: inner diameter: 3mm/0.12in, outer diameter: 4mm/0.16in Suitable for common air pipe(innner diameter approx. 4mm/0.16in) Note: The OUT surface is air outlset, and the other side is air inlet. Package includes: 5*Non-Return Check Valve Notes: Transition: 1cm=10mm=0.39inch Please allow 1-3mm error due to manual measurement. pls make sure you do not mind before you bid. Due to the difference between different monitors, the picture may not reflect the actual color of the item. Thank you