breakage of the ring and perhaps of the land is almost certain if the ridge is not removed. PISTON  PINS  AND  PIN  BEARINGS Piston pins are made of hardened steel alloy, and their surfaces are precision finished. Piston sleeve bearings or bushings are made of bronze or a similar material. These pins and pin bear- ings require very little service and total failure seldom  occurs. Wear,   pitting,   and   scoring   are   the   usual troubles  encountered  with  piston  pins  and  piston pin  bearings. Wear of a pin or bearing is normal, but the rate of wear can be unnecessarily increased by such  factors  as  inadequate  and  improper  lubrica- tion,   overloading,   misalignment   of   parts,   or failure of adjacent parts. Every time a piston assembly is removed from an  engine,  the  complete  assembly  should  be checked for wear. Piston pins and bushings should be measured with a micrometer to determine if wear is excessive. Do NOT measure areas that do not make contact, such as those between the con- necting rod and piston bosses, and the areas under the oil holes and grooves. The correct and limiting values  for  measurements  may  be  found  in  the manufacturer’s  technical  manual  for  the  par- ticular  engine. Excessive wear of pins, bushings, or bearings is  often  the  result  of  insufficient  or  improper lubrication.  (These  parts  are  usually  pressure lubricated.)  The  failure  of  a  pressure  lubricating system  is  usually  detected  before  piston  pins, bushings,  or  bearings  are  seriously  damaged. Insufficient lubrication of these parts is usually caused  by  obstructions  blocking  the  oil  passages of the connecting rods. If the bushings have been installed  so  that  the  oil  holes  do  not  line  up, lubrication may be restricted. Such misalignment of oil holes may also be caused by a bushing com- ing  loose  and  revolving  slightly  out  of  position. Also interchanging the upper and lower connect- ing  rod  bearings  ON  SOME  ENGINES  may obstruct the flow of oil to the upper end of the rod. Always check the manufacturer’s technical manual for information on interchangeability of parts. If  there  is  misalignment  of  the  connecting rods, uneven loading on piston pins and bearings will result. The fact that a rod is misaligned is usually indicated by uneven wear of the piston pin and  bushing  and  by  piston  skirt  wear.  Misalign- ment may be caused by improper reaming of the bushing for proper clearance. CONNECTING RODS Connecting rod troubles usually involve either the connecting rod bearing or the piston pin bear- ing. Some of these troubles, such as misalignment, defective bolts, cracks, or plugged oil passages, can be avoided by performing proper maintenance and  by  following  instructions  in  the  manufac- turer’s technical manual. Misalignment causes binding of the piston, piston pin, and the connecting rod journal bear- ing. This binding is likely to result in breakage and in increased wear of the parts, leading to total failure  and  possible  damage  to  the  entire  engine structure. Connecting rods must be checked for proper  alignment  before  being  installed  in  an engine,  and  after  any  derangement  involving  the piston,  cylinder,  or  crankshaft. Defective bolts are often the result of over- tightening.   Connecting   rod   bolts   should   be tightened   by   using   a   torque   wrench,   or   an elongated gage to ensure that a predetermined turning  force  is  applied  to  the  nut.  Defective threads  can  cause  considerable  trouble  by  allow- ing  the  connecting  rod  to  be  loosened  and  cause serious damage to the engine. Whenever rod bolts are removed they should be carefully inspected for  stripped  or  damaged  threads  and  elongation. Cracked  rods  are  usually  the  result  of overstressing   caused   by   overloading   or overspeeding or because defective material was used at the time of manufacture. It is of prime importance to discover the cracks before they have developed to the point where the failure of the rod will take place. No attempts should be made to repair cracked rods. They should be replaced; serious  damage  may  result  if  breakage  occurs during operation. Restricted oil passages are often the result of improper assembly of the bushing and the con- necting rod bearing inserts. They may also be due to foreign matter lodging in the oil passages. ENGINEMAN  1  &  C 3-30