BikeMaster Tube - Straight Metal Stem (3.75-4.25-18) BikeMaster Tubes are made from high-quality butyl rubber, utilize heavy-duty valves, and are manufactured using a very durable seamless construction. It's the perfect choice to get your bike rolling smooth again! Seamless construction. Best inner tube value available. Butyl rubber inner tube with heavy-duty valve. Tube Specifications: Tube size: 3.75-4.25-18.