TM 1-1520-240-BD (c) Stiffeners are not permitted in areas where they will interfere with installation and function or operation of any component. (3) Determine   locations   where   a   minimum number of stiffeners will correct oilcanning or loose skin. (4) Add   stiffeners   as   required.      Stiffeners must be attached at each end to primary structure, such as longeron to longeron or frame to frame.  The method of    attachment    should    eliminate    discrepancy    without transferring  it  to  another  area.    Comply  with  following methods: (a) Stiffeners    must    be    next    heavier thickness  than  skin  to  which  they  are  attached  and  are to    be    formed    from    2024-T3    cad    aluminum    alloy. Stiffeners exceeding 30 inches in length must be made from Alcoa 33372 or equivalent extrusions. (b) If   attaching   clips   are   used,   make from 2024-T2 dad aluminum alloy.  They must be same thickness as stiffener to which they are attached. (c) Install rivets MS20470AD4 at approximately 3/4-inch pitch for attachment of stiffeners, not thicker than 0.040 inch. (d) Install rivets MS20470AD5 at approximately 7/8-inch pitch for attachment of stiffeners thicker than 0.040 inch. (e) Seal and refinish stiffeners and clips to match adjacent structure. CAUTION Substitution of materials can create a corrosion  hazard.    If  the  substitute material is different than original part (as when a 7075-T6 aluminum extrusion  is  reinforced  with  a  4130 steel strap), failure to insulate metals from  direct  contact  with  each  other will  permit  electrolytic  corrosion  to attack   repaired   area.   Insulation   of dissimilar metals, is extremely important. b. Material Substitution.  If repair cannot be made from    the    same    material    as    original    part,    select    a substitute   material.      Refer   to   Table   2-6   for   a   list   of substitute materials and an example of how to use this table.  When material for a non-load-bearing part, such as a filler, is not available in thickness required, build up to desired thickness with two or more pieces. Substitutions of this type are not permitted-for loadbearing parts, such as reinforcements and insertions. 2-17.  SKIN  AND  WEB  REPAIRS.    (See  Figure  2-34.) Subsequent paragraphs contain procedures and limitations  for  coin  patch  repair  for  skins,  backing  plate repair  for  webs.    (See  TM  1-1500-204-23  for  rivet  type and  spacing.)    Maintain  a  minimum  clearance  of  5/8 inch between cutouts and edge of replacements, unless otherwise noted.  Insulate dissimilar metals. a. Coin  Patch  Repair  for  Skins.    This  repair  is  for dents, punctures, tears, and small cracks that can be cut out from 1/2 inch to 1 1/2-inch diameter.  No more than two patches are permitted for each skin panel.  Panels, not less than 15 by 50 inches, between longer longerons and  sta  160  and  482,  may  contain  a  maximum  of  four patches.      All   patches   must   clear   adjacent   structural members by 1/8 inch and be sealed. (1) Cut out damage and make a coin patch of same material and thickness as skin.  The patch should have a diameter 5/16 inch less than cutout. (2) Make a circular backing plate three times diameter  of  coin  patch  and  from  same  material  but  in next heavier thickness. (3) Smooth  edges  of  cutout,  coin  patch,  and backing plate with a file. (4) Center  coin  patch  on  backing  plate  and drill rivet holes through coin patch and backing plate. NOTE The  size  and  number  of  rivet  holes are determined by size of rivet to be used. In most cases rivets MS20470AD3 can by used. (5) Install and set rivets. (6) Spray or brush a thin coat of primer (item 113,  Appendix  C)  on  all  surfaces  of  coin  patch.    Allow primer to dry. 2-59