tolerate temperature extremes in excess of those encountered  in high pressure hydraulic systems. Their dash numbers  indicate not only their size but also relate directly to the dash  number of the O-ring for which they are dimensionally suited.  They are procurable under a number of basic part numbers,  but they are interchangeable; any Teflon$^{™}$ backup ring may  be used to replace any other Teflon$^{™}$ backup ring if it is of  proper overall dimension to support the applicable O-ring.  Backup rings are not color coded or otherwise marked and  must be identified from package labels. The inspection of  backup rings should include a check to ensure that surfaces  are free from irregularities, that the edges are clean cut and  sharp, and that scarf cuts are parallel. When checking Teflon™  spiral backup rings, make sure that the coils do not separate  more than ¼ inch when unrestrained. Be certain that backup  rings are installed downstream of the O-ring.  [Figure 12-59] and its cross-section is small in relation to its diameter.  The cross-section is truly round and has been molded and  trimmed to extremely close tolerances. The O-ring packing  seals effectively in both directions. This sealing is done by  distortion of its elastic compound. Advances in aircraft design have made new O-ring composition  necessary to meet changing conditions. Hydraulic O-rings  were originally established under Air Force-Navy (AN)  specification numbers 6227, 6230, and 6290 for use in fluid  at operating temperatures ranging from –65 °F to +160 °F.  When new designs raised operating temperatures to a possible  +275 °F, more compounds were developed and perfected.  Recently, newer compounds were developed under Military  Standard (MS) specifications that offered improved  low-temperature performance without sacrificing high- temperature performance. These superior materials were  adopted in the MS28775 O-ring, which is replacing AN6227  and AN6230 O-rings, and the MS28778 O-ring, which  is replacing the AN6290 O-ring. These O-rings are now  standard for systems where the operating temperatures may  vary from –65 °F to +275 °F. Gaskets are used as static (stationary) seals between two flat  surfaces. Some of the more common gasket materials are  Manufacturers provide color coding on some O-rings, but  this is not a reliable or complete means of identification.  The color coding system does not identify sizes, but only  system fluid or vapor compatibility and, in some cases, the  manufacturer. Color codes on O-rings that are compatible  with MIL-H-5606 fluid always contains blue but may also  contain red or other colors. Packings and gaskets suitable for  use with phosphate ester-based fluids are always coded with  a green stripe, but may also have a blue, grey, red, green, or  yellow dot as a part of the color code. Color codes on O-rings  that are compatible with hydrocarbon fluid always contain  red, but never contain blue. A colored stripe around the  circumference indicates that the O-ring is a boss gasket seal.  The color of the stripe indicates fluid compatibility: red for  fuel, blue for hydraulic fluid. The coding on some rings is not  permanent. On others, it may be omitted due to manufacturing  difficulties or interference with operation. Furthermore,  the color coding system provides no means to establish the  age of the O-ring or its temperature limitations. Because of  the difficulties with color coding, O-rings are available in  individual hermetically sealed envelopes labeled with all  pertinent data. When selecting an O-ring for installation, the  basic part number on the sealed envelope provides the most  reliable compound identification. Backup rings (MS28782) made of Teflon™ do not deteriorate  with age, are unaffected by any system fluid or vapor, and can  12-37 Figure 12-59.  Backup O-rings installed downstream.  Backup Rings O-Ring Color Coding  Gaskets A B C No cut solid Spiral cut Single turn (scarf-cut) Backup rings are installed downstream of  some O-rings to prevent extrusion Orientation of a spiral cut teflon backup ring Wrong Backup ring O-ring Correct Correct after installation O-ring without back up ring extrudes into gap Fluid pressure