CO2 cylinders that match with your refilling station. Order them from us and we will deliver them to your home quickly and reliably (4 cylinders/order). Purchase cylinders: The purchase cylinders are more expensive than the replacement cylinders because they include a deposit fee. Please keep the shipping box such that you can use it to return the empty purchase cylinders for new replacement cylinders. We will send you a return label together with the purchase cylinders. Replacement cylinders: If you've already ordered 4 purchase cylinders from us, then use the shipping box and the enclosed return label to return the 4 empty purchase cylinders to us after ordering the replacement cylinders. As soon as we receive the cylinders, we will deliver the new cylinders to your home. Thread type "TR 21x4": Standard type, which is also known from other soda makers.