TM55-2640-241-23 5-17.   Fuel   System   Tubes. a.    Removal    Observe    the    following    precautions when    removing    tubes. (1)   When   loosening   a   coupling   nut,   always   use a backup wrench on the mating fitting to prevent the fitting    from    turning. (2) Always use the proper size wrench to pre- vent   rounding   off   the   hex   corners. (3)   Attach   th e  wrench   to   the   fitting   or   coupling nut  in  a  manner  that  will  prevent  crushing  of  the  fitting or   coupling   nut. (4)  Never  use  a  pipe  wrench,  pliers,  or  vise grips   on   a   coupling   nut   or   fitting. b . Cleaning. Flush    tubes    with    solvent    (item    3,    table 2-2)   and   blow   dry. c . Inspection.  Inspect  and  reject  tubes  for  the  fol- lowing damage: (1)  Rinks  or  en  s  which  could  obstruct  fuel d     t flow.   Dents   are   allowable   up   to   0.015   in.. (2)   Cracked   or   broken   tubes   or   coupling   nuts.   If cracked   tubes   or   coupling   nut   is   suspected,   refer   to   TM l-1520-254-23,     Technical     Manual     Aviation     Unit     Main- tenance  (AWM)  and  Aviation  Intermediate  Mainte- nance    (AVIM)      Manual    Nondestructive    Inspection Procedure for OH-58 Helicopter Series. (3)    Cross-threaded, crushed,   or   otherwise damages     coupling     nuts. (4)  Chafing  within  clamp  areas  in  excess  of 0.010 in. deep. Chafing is not allowed at or near the flared    tubing    end. (5)   Nicks   which   exceed   0.010   in.   deep. d. Installation. (1) Observe the precautions listed in removal when    installing    the    fuel    tubes. (2)   Tighten  No.  4  size  coupling  nuts  80  to  120  in. lb.   and   No.   5   size   coupling   nuts   150   to   200   in.   lb. (3)   Rigid   steel   tubing   must   align   with   its   mat- ingfittings  in  the  free  state  such  that  both  coupling  nuts will  engage  two  full  threads  of  their  mating  fittings with   light   finger   pressure.   The   tube   must   be   stretched during  final  tightening  of  the  coupling  nuts.  Adjust- ments  may  be  made  by  bending  the  tube  at  the  principal bend   or   bends.   All   bending   must   be   accomplished   with the tube removed from the engine. flattening effect as a   result   of   reforming   shall   not   exceed   15   percent   of   the tube    OD.. 5-17.1.  Fuel  Check  Valve. a.     Removal. (1)   Disconnect   Fire   Shield   to   Fuel   Nozzle   Hose (5,   Figure   5-3)   from   fuel   nozzle   (6)   and   fuel   check   valve (1). (2)  Disconnect  fuel  control-to-firewall  shield tube   (4)   from   Fuel   Check   Valve   (1). (3)   Remove   nut   (3)   washer   (2)   and   check   valve (1)   from   the   firewall      shield. b.     Inspection. Visually    inspect    general    condition    of    valve    and check   associated   tubing   for   cracked   or   broken   tubes   or coupling   nuts.   If   cracked   tubes   or   coupling   nut   is   sus- pected,  refer  to  TM  l-1520-254-23,  Technical  Manual Aviation   Maintenance   (AWM)   and   Aviation   Inter- mediate Maintenance (AVIM)   Manual Nondestructive Inspection Procedure for OH-58 Helicopter Series. c. Installation. (1)   Install   the   check   valve   (1)   and   washer   (2)   in the  fireshield.  Install  from  the  aft  side  with  arrow pointing  rearward.  Secure  the  valve  with  washer  (2) and   nut   (3).   Tighten   55-80   in.   lb. (2) Install fuel control to check valve tube (4) and   fuel   nozzle   to   check   hose   (5).   Tighten   coupling   nuts to 80-120 in. lb. Section    Ill.    LUBRICATION   SYSTEM Failure   to   properly   install,   align,   and tighten  oil  fittings  and  tubes  could  re- sult  in  an  engine  failure. 5-18. Scope. Lubrication    system    servicing    includes    cleaning    and inspecting   the   magnetic   chip   detectors   and   oil   pressure regulator,   changing   the   system   oil,   cleaning   the   oil   fil- ter,   removing   and   reinstalling   the   oil   filter   housing,   oil pressure   reducer,   and   lube   oil   check   valves.   In   addition, it includes adjusting the oil pressure regulating valve and   the   scavenge   oil   flow   check. 6-19. Use of Oils. NOTE It is not advisable to mix MIL-L-7807 and MIL-L-23699 oils except in cases of emer- gency. If it becomes necessary, the system must   be   flushed   within   6   hours   of   operation. (Refer to paragraph 5-21.) NOTE Nearly   all   present   day   lube   oils   contain   an- tifoam   additives   which   can   settle   to   the   bot- tom  of  the  container.  Thoroughly  shake (agitate)   the   container   prior   to   adding   oil   to the   system   either   at   an   oil   change   or   during routine servicing. a . Use  of  MIL-L-23699  oil  is  authorized  and  di- rected    for    ambient    temperatures    above    minus    25°F. 5-16    Change    21