Failure of an O-ring can sometimes begin with the  removal  of  an  old  O-ring.  If  you  incorrectly remove  an  O-ring  with  pointed  or  sharp  tools,  you can scratch or dent critical surface finishes that can result in seal failure. Before  installing  a  new  O-ring,  inspect  the sealing surfaces for any abrasions and wipe them free  of  any  dust,  dirt,  or  other  contaminants. Before  installation,  inspect  the  O-ring  for  any damage.  If  faulty,  discard  it. When  you  install  the  O-ring,  lubricate  it.  In most  cases  it  is  already  coated  with  the  system fluid  or  petrolatum  grease.  Do  not  stretch  the O-ring  more  than  twice  its  original  size  during installation,   and   do   not   roll   or   twist   it   into place.  This  may  leave  a  permanent  twist  in  the O-ring  and  reduce  its  effectiveness  and  shorten its life. When  installing  an  O-ring,  take  extreme care  to  avoid  forcing  it  over  sharp  edges, corners,  and  threaded  sections.  You  should  use some  type  of  sleeve  or  cover  to  avoid  damaging the  O-ring. FASTENERS The proper use of fasteners is very important and cannot be overemphasized. Many shipboard machinery  casualties  have  resulted  from  fasteners that  were  not  properly  installed.  Machinery vibration, thermal   expansion,   and   thermal contraction   will   loosen   the   fasteners.   At   sea, loosening effects are increased by the pitch and roll   of   the   ship.   You   are   familiar   with   such standard   fasteners   as   nuts,   bolts,   washers, wingnuts,  and  screws.  In  this  section  we  will discuss  some  of  the  new  developments  in  fastener technology, such as the various types of locknuts, which  you  may  not  be  familiar  with. THREADED  LOCKING  DEVICES An important part of fastener technology has included the development of several methods for locking mated threads of fasteners. Many of the latest   methods   include   the   locking   device   or method  as  an  integral  part  of  the  fastener assembly   and   are   referred   to   as   self-locking nuts  or  bolts.  Self-locking  fasteners  are  more expensive than some older methods but compare favorably  in  cost  with  pin  or  wiring  methods. Length of Protrusion Male  threads  on  threaded  fasteners,  when installed  and  tightened,  will  protrude  the  distance of   at   least   one   thread   length   beyond   the top  of  the  nut  or  plastic  locking  ring.  Excessive protrusion  is  a  hazard,  particularly  where necessary clearances, accessibility, and safety are important.   Where   practicable,   the   number   of threads protruding should not exceed five. In no case should thread protrusion exceed 10 threads unless  specifically  approved  by  the  work  super- visor.  (This  is  the  1-to-10  rule.) Where screw threads are used for setting or adjusting (such as valve stem packing glands and travel  stops)  or  where  installed  threaded  fasteners do  not  strictly  follow  the  1-to-10  rule  but  have given  satisfactory  service,  the  rule  does  not apply.   An   example   of   an   acceptable   existing installation would be where a male thread is flush with  the  top  of  a  nut  or  where  more  than  10 threads  protruding  is  of  no  foreseeable  con- sequence. Repair of Damaged Threads You can remedy damaged external threads by replacing the fastener. In large equipment castings you  must  repair  damaged  internal  threads  to  save the  part.  You  can  repair  internal  threads  by redrilling the damaged thread; clean and either install a solid wall insert or tap for a helical coil insert. These inserts, in effect, return the tapped hole  to  its  original  size  so  it  takes  the  original mating  fastener. LOCKNUTS Locknuts   are   used   in   special   applications where  you  want  to  ensure  that  the  components joined by the fasteners will not loosen. Two types of  locknuts  are  in  common  use.  The  first  type applies  pressure  to  the  bolt  thread  and  can  be used  where  frequent  removal  may  be  required. The  second  type  deforms  the  bolt  thread  and  is used  only  where  frequent  removal  is  unnecessary. The  first  type  includes  plastic  ring  nuts,  nylon insert  nuts,  jam  nuts,  spring  nuts,  and  spring beam  nuts.  The  second  type  includes  distorted collar nuts and distorted thread nuts; they are not commonly  found  in  gas  turbine  equipment  and will  not  be  covered  in  this  section. 9-45