An O-Ring functions as a seal through the mechanical deformation of the elastomeric compound by mating metal surfaces. The resilient rubber conforms to the shape of the metal sections and blocks the passage of gases or liquids. If the pressure increases, the O-Ring is further deformed and provides tighter sealing. APG O-Ring, Grade A, Series: V, 77 mm Inside Dia, Cross Section: 2.5 mm, 75 Durometer, -15 to 400 °F, FKM, Black, Import