Kenda manufactures 31 different size butyl rubber ATV tubes to fit all of today's popular ATV sizes. Kenda tire tubes will not add unsprung weight to your wheels and will keep you up and running when it counts. Engineered to keep you on the right path by offering dependable tubes for various applications from industrial to personal - Kenda stands by its commitments to deliver high-quality tubes and tires at a price you can easily afford. Tube Specifications: Size: 21 x 7-10. Valve type: TR-6 (straight metal stem (8 mm diameter)).