The O-ring is a popular pressure and fluid seal. The seal between overlapping, cylinder-shaped matching surfaces and are engine parts, plumbing fixtures, flanges, valves, and other mechanical components cylinders. They have a round outline and a circular cross section. Similar to a doughnut's central hole. A variety of sizes and materials are available for O-rings. That available in a range of sizes and custom-ordered to match the nominal size. The thickness (TH), cross-sectional area (CSA), and diameter (ID, OD, and OD, respectively).