Back-up rings are used to prevent O-rings from extruding when the tight tolerances listed in the previous section cannot be maintained. Back-up rings do not provide any sealing function. They are simply intended to reduce the extrusion gap on the low-pressure side so that the O-ring can fulfill its sealing function without being damaged. Back-up rings are made of materials with better extrusion resistance than most elastomers. Several common materials are high durometer NBR (or other elastomers), nylon and filled PTFE. Since the materials used for back-up rings are basically non-compressible and non-deformable, they can typically be treated as though they are part of whatever the O-ring groove is machined into. That is, the width of the groove has to be increased just enough to accommodate the thickness of the back-up ring. For back-up rings with a curved surface facing the O-ring, the effective thickness of the back-up ring can be determined by dividing the cross-sectional area of the back-up ring by the height of the back-up ring. This ensures that the O-ring has the same area to occupy. DICHTOMATIK Back-Up Ring, 4.778 in Inside Dia, 0.183 in W, 0.076 in Thickness, Contoured Cross Section, Dash Number: 8-351, 90 Shore A Durometer, -40 to 250 °F, Buna-N