TM 1-1500-204-23-1 bearings are used where the bearing is subjected to radial loads only In tapered roller bearings, the inner and outer- race  bearing  surfaces  are  cone  shaped.    Such  bearings will withstand both radial and thrust loads.  Straight roller bearings  are  used  in  high-power  aircraft  engines  for  the crankshaft  main  bearings.    They  are  also  used  in  other applications where radial loads are high. (2) Maintenance   Clean,   inspect,   and   replace beanngs.  in  accordance with the applicable maintenance manual d. Connecting Rods.  The connecting rod is the link which    transmits    forces    between    the    piston    and    the crankshaft.    Connecting  rods  must  be  strong  enough  to remain rigid under load and yet be light enough to reduce the  inertia  forces  which  are  produced  when  the  rod  and piston stop, change direction, and start again at the end of each stroke. (1) Types  There  are  three  types  of  connecting rod  assemblies,  as  shown  in  figure  7-9.    The  plain,  fork- and-blade,    and    master-and-articulated    connecting    rod assemblies are described in the following paragraphs. (a) Plain connecting rods. Plain connecting rods are used In opposed engines.  The end of the rod attached to the crankpin Is fitted with a cap and a two-piece beanng.  The beanng cap is held on the end of the  rod  by  bolts  or  studs.    To  maintain  proper  fit  and balance,  connecting  rods  should  always  be  replaced.    In the same cylinder and In the same relative position. (b) Fork-and-blade  connecting  rods  The fork-and-blade  rod  assembly  Is  used  primarily  in  V-type engines  The  forked  rod  is  split  at  the  crankpln  end  to allow space for the blade rod to fit between the prongs.  A single two-piece bearing is used on the crankshaft end of the rod. (c) Master-and-articulated connecting rods.    The  master-and-articulated  rod  assembly  is  only used  in  radial  engines.    In  a  radial  engine  the  piston  in one cylinder in each row is connected to the crankshaft by a master rod.  All other pistons.  In the row are connected to the master rod by an articulated rod. (2) Maintenance. The maintenance of connecting   rods   involves   visual   Inspection   for   nicks, cracks,  bending,  corrosion,  and  other  damage,  magnetic particle inspection, checking alignment for parallelism and convergence between the bearing end and the piston end; rebushing, and replacement of the bearings.  Refer to the applicable maintenance manual before accomplishing any maintenance procedures. e. Pistons.   Pins.   and   Rings.      Pistons,   pins,   and rings,    as    shown    in    figure    7-10,    are    described    and maintained as explained In the following paragraphs. (1) Pistons.      The   piston   of   a   reciprocating engine  is  a  cylindrical  member  which  moves  back  and forth within a steel cylinder.  The piston acts as a moving wall within the combustion chamber.  As a piston moves down  the  cylinder,  it  draws  In  the  fuel-air  mixture.    As  it moves upward, it compresses the charge, ignition occurs, and the expanding gases force the piston downward.  This force    is    transmitted    to    the    crankshaft    through    the connecting  rod.    On  the  return  upward  stroke,  the  piston forces the exhaust gases from the cylinder. (2) Pins.  The piston pin joins the piston to the connecting rod It Is machined in the form of a tube from a nickel steel alloy forging, casehardened and ground.  The piston  pin  is  sometimes  called  a  wristpin  because  of  the similarity  between  the  relative  motions  of  the  piston  and the articulated rod and that of the human arm. (3) Rings.  The piston rings prevent leakage of gas pressure from the combustion chamber and reduce to a   minimum   the   seepage   of   oil   into   the   combustion chamber.  The rings fit into the piston grooves but spring out   to   press   against   the   cylinder   walls,   when   properly lubricated, the rings form an effective gas seal.  Rings are always used in sets. (4) Maintenance.    Pistons      and      pins      are Inspected   and   checked   for   cleanliness,   wear,   scoring, corrosion,  cracks,  and  any  apparent  damage  New  rings are.  Installed each time the engine is overhauled.  Refer to the applicable maintenance manual when accomplishing maintenance on pistons, pins, and rings. f. Cylinders.  The portion of the engine in which the power  is  developed  is  called  the  cylinder.    The  cylinder provides  a  combustion  chamber  where  the  burning  and expansion of gases take place, and.  It houses the piston and the connecting rod (1) Description.       The    cylinder    used    In    the aircooled   engine   is   the   overhead   valve   type   shown   in figure   7-11.      Each   cylinder   assembly   consists   of   the cylinder head and the cylinder barrel. 7-6