TM 1-1500-204-23-1 5 Position rocker box cover mounting surface  down  on  lapping  plate,  and  use  a  figure  eight motion to keep an even pressure on cover. 6 When warpage cannot be eliminated without exceeding allowable limits given in the applicable    maintenance    manual,    replace    rocker    box cover. 7 Replace    leaking    seals    with    like serviceable items. 8 Replace individual piston rings that are damaged upon installation of a complete cylinder and piston   assembly.      Refer   to   illustrated   parts   breakdown manual of applicable engine for correct size and type ring. 9 Replace  cracked  or  broken  rocker arms. 10 Replace bent pushrods. 11 Replace   bent   or   cracked   pushrod housing. (i) Cylinder  replacement.    Cylinders  will be replaced for the following reasons as described in the applicable maintenance manual. Low compression High oil consumption Excessive valve guide clearance Loose intake pipe flanges Loose  or  defective  spark  plug  inserts  (steel  or brass) External damage, such as cracks Spark plug copper runout NOTE Engines  that  have  spark  plug  copper runout on more than 25 percent of the cylinders will be removed from service.  Carbon deposits must not be confused with copper runout g. Valves.  The fuel-air mixture enters the cylinders through   the   intake   valve   ports,   and   burned   gases   are expelled  through  the  exhaust  valve  ports.    The  head  of each  valve  opens  and  closes  these  cylinder  ports.    The valves   used   In   aircraft   engines   are   the   conventional poppet  type.    The  valves  are  also  typed  by  their  shape and are called either mushroom or tulip because of their resemblance  to  the  shape  of  these  plants  Figure  7-14 shows various shapes and types of these valves. (1) Description.  The valve head has a ground face which forms a seal against the ground valve seat in the  cylinder  heads  when  the  valve  is  closed.    The  valve stem  acts  as  a  pilot  for  the  valve  head  and  rides  in  the valve guide installed in the cylinder head for this purpose. The  valve  stem  is  surface-hardened  to  resist  wear  The neck is the part that forms the junction between the head and   the   stem.      The   tip   of   the   valve   is   hardened   to withstand  the  hammenng  of  the  valve  rocker  arm  as  it opens  the  valve.    A  machined  groove  on  the  stem  near the tip receives the split-ring stem keys These stem keys form a locking to hold the valve spring retaining washer in place (2) Maintenance.  Proper valve operation is of vital  importance  to  satisfactory  operation  of  the  engine from the standpoint of stability and cylinder wear.  Valve clearances   establish   valve   timing   when   the   engine   is operating,   and   since   cylinders   receive   fuel-air   mixture from a common supply, the valve clearances should be Figure 7-14.  Valve Types 7-14