The standard U-cup, which has a square cross section, is known as the 6226 Series. They are typically made with 80 Buna-N, although other compounds are available on request. The 6226 Series are for general purpose, low pressure air and hydraulic use. Compounds from neoprene exhibit fair oil resistance, good ozone & weather resistance. U-cups are available in various shapes, size ranges and homogeneous or fabric reinforced compounds. U-cups are most often used for sealing rams or rods, although they also can be adapted to piston sealing applications. The U-cup selection should be made on the basis of inside or outside sealing, operating pressure, temperature, and the type of air or fluid medium in which the cup must function. 70-75 Buna-N Durometer.