TM 1-1500-204-23-1 i. Low  Cylinder  Compression.    Prior  to  removing cylinders  for  low  compression,  check  and  adjust  valve clearance.      Check   cylinder   compression.      Replace cylinder   when   compression   is   less   than   established limits. j. Parts  Failure.    When  some  portion  of  a  steel part, such as a piston ring, valve, valve spring, washer, rocker arm, or rocker arm bearing cannot be located and removed,  the  engine  will  be  replaced,  so  other  parts within the engine will not be damaged. k. Basic   Mechanical   Failure.      Remove   engine when  any  internal  component  such  as  impeller,  boss, casting,   propeller   shaft,   or   studs,   become   cracked, nicked,   broken,   or   damaged   to   the   extent   that   safe operation cannot be continued.  Failures of this type are limited  to  Internal  components  of  the  engine  such  as impeller,    Impeller    clutch,    crankshaft,    supercharger housing,   and   reduction   gear   housing,   since   external failures, such as cylinders or engine accessories, can be corrected by replacing the unserviceable assembly. I. Engine    Overspeed.        When    engine    speeds exceed  limits  specified  in  the  applicable  maintenance manual, the removal or inspection of the engine will be dependent   upon   the   amount   of   overspeed   reported. When   engine   speeds   exceed   limits,   accomplish   the following procedures (1) When overspeed is above maximum rpm, replace engine. (2) When overspeed is below maximum rpm, accomplish the following: (a) Examine  engine  for  cracked,  open, or broken cylinder heads and barrels, and bent push rod housing. NOTE In    the    event    a    cylinder    head    is cracked or broken and no particles of pistons rings have entered crankcase of  engine  (determined  by  Inspection of magnetic sump plug or oil screen for metal), remove damaged cylinder and  piston  and  examine  articulating and master rods for misalignment.  If rods are satisfactory, replace cylinder  and  piston  assembly  with  a serviceable unit and continue engine in service as specified below. (b) Examine oil system.  When examination of oil  screens  or  filters  discloses  an  abnormal  amount  of metal    particles,    remove    engine.        When    no    metal particles are found, operate engine as specified. (c) When examination of engine discloses no visible damage as a result of overspeeding, restrict aircraft to local flights for a period of 10 hours. m. Excessive Manifold Pressure.               Engines incorporating    turbosuperchargers    or    auxiliary    stage superchargers     are     subject     to     excessive     manifold pressure when automatic controls malfunction or fall.   A manifold pressure overboost control is usually employed to    reduce    the    possibility    of    overboost    occurring. However, when overboost occurs, combustion temperatures  rise,  and  detonation  or  pre-ignition  may result   and   cause   serious   structural   damage   to   the engine.        When    manifold    pressure    exceeds    value specified   in   the   flight   manual,   it   Is   considered   an overboost condition and the engine will be removed. n. Excessive  Maintenance.    If  an  aircraft  engine requires   consistent   excessive   maintenance   or   is   a definite hazard to flight, it should be replaced, provided complete    engine    conditioning    will    not    correct    the difficulty.  Excessive and consistent oil fouling of spark plugs could cause excessive maintenance.  However, if fouling  is  limited  to  certain  cylinders,  the  difficulty  may be  eliminated  by  replacing  defective  cylinders,  ignition leads,  distributor  block,  or  opening-clogged  rocker  box drains. o. Excessive Oil Consumption.  Oil is consumed in the engine by being burned in the combustion chamber, lost  in  the  form  of  mist  or  vapor  from  the  breather,  or leakage where propeller shafts and other shafts protrude from  the  crankcase.    Consumption  varies  with  engine power  setting,  however,  one  or  more  cylinders  fouling due  to  excessive  oil  consumption  may  call  for  cylinder rather than engine change. (1) Oil   consumption.      Some   of   the   factors which    affect    oil    consumption    are    engine    speed, temperature, wear condition, and lubricant characteristics such as viscosity-temperature coefficient, volatility,   and   flash   point.      Each   is   explained   in   the following paragraphs (a) Engine speed.  The most important factor   affecting   oil   consumption   normally   is   engine speed. oil pressure Increases with engine speed until the ,  operating  range  pressure  of  the  engine  is  reached, then   a   pressure   relief   valve   maintains   a   constant pressure.   Usually,   maximum   pressure   is   reached   at relatively low speeds.  The effect of engine speed on oil pressure 7-28