Buna N O'rings are one of the most common and basic seals used because of their resistance to water and petroleum based fuels & lubricants. Under pressure O'Rings stretch and flow, filling seal cavity and blocking passage of liquids and gases, but capable of recovering their original shape after being stretched. At extremely low temperatures O'Rings become brittle but still function as a seal. Upon warming the O'Ring will assume its flexible properties without harm. Prolonged exposure to excessive heat will cause O'Rings to permanently harden and ultimately fail. Buna N O'rings are not recommended for exposure to sunlight, UV light or general outdoor weathering. O'Rings are measured by their inside diameter and the thickness of the O'Ring.