This fresh water gravity fill hatch is designed to fill RV tanks without the help of pressure or threaded wire. The diameter of gravity inlet hatch is 4-1/2" and universally fits for 3-1/2" cutouts. And the actual barbed connector has two stages to accept filling hoses with inner diameters between 1-1/4" and 1-3/8". Perfect for most models of RVs. The gravity inlet hatch is made of food-grade ABS. This ABS plastic resists UV and fade, making the hatch durable and pretty. The 1/2" air vent/overflow on the hatch will increase the flow rate by preventing air locks. Bayonet-style caps are easier to screw on and off. The cap tether ensures ‌you won’t lose the fill cap. RVGUARD RV Fresh Water Hatch, Gravity Fill Hatch Inlet With 1-1/4" Barbed Connector and 1/2" Air Vent, for RV Tanks, Caravan, Camper (Black)