be  burned  in  the  cylinder.  This  results  in  the accumulation of excessive carbon deposits. Crown and Land Dragging Pistons and liners may become sufficiently worn to permit  the  piston  to  cock  over  in  the  cylinder.  This allows the crown and ring lands to drag on the cylinder wall.  The  results  of  dragging  can  be  determined  by visually inspecting the parts of the piston in question. Piston  Wear Although piston wear is normal in all engines, the amount  and  rate  of  piston  wear  depend  on  several controllable  factors.  (The  causes  of  excessive  piston wear, and crown and land dragging, are also the causes of other piston troubles.) One of the controllable factors is LUBRICATION. An adequate supply of oil is essential to provide the film necessary to cushion the piston and other parts within the   cylinder   and   prevent   metal-to-metal   contact. Inadequate lubrication will not only cause piston wear and crown and land dragging, but also may cause piston seizure, and piston pin busing wear. Lack of lubrication is caused either by a lack of lube oil   pressure   or   by   restricted   oil   passages.   The pressure-recording instruments usually give warning of low oil pressure before any great harm results. However, clogged  passages  offer  no  such  warnings,  and  their discovery  depends  on  the  care  that  is  exercised  in inspecting and cleaning the piston and connecting rod assembly. Another controllable factor that may be directly or indirectly  responsible  for  many  piston  troubles  is IMPROPER COOLING WATER TEMPERATURE. If  an  engine  is  not  operated  within  the  specified temperature  limits,  lubrication  troubles  will  develop. High  cylinder  surface  temperatures  will  reduce  the viscosity of the oil. As the cylinder lubricant thins, it will run  off  the  surfaces.  The  resulting  lack  of  lubrication leads  to  excessive  piston  and  liner  wear.  However,  if temperatures  are  below  those  specified  for  operation, viscosity will be increased, and the oil will not readily reach the parts requiring lubrication. Oil  plays  an  important  role  in  the  cooling  of  the piston  crown.  If  the  oil  flow  to  the  underside  of  the crown is restricted, deposits caused by oxidation of the oil will accumulate, lowering the rate of heat transfer. Therefore, the underside of the piston crown should be thoroughly cleaned whenever pistons are removed While insufficient and uneven cooling may cause ring  land  failure,  excessive  temperatures  may  cause piston seizure; an increase in the rates of oxidation of the oil, resulting in clogged oil passages; or damage to piston  pin  bushings. Seizure or excessive wear of pistons may be caused by  IMPROPER  PIT.  New  pistons  or  liners  must  be installed   with   the   piston-to-cylinder   clearances specified in the manufacturer’s instruction manual. PISTON PINS AND SLEEVE BEARINGS OR BUSHINGS Every time you remove a piston assembly from an engine, inspect it for wear. Measure the piston pins and sleeve bearings or bushings with a micrometer, as shown in figure 3-25, to determine whether wear is excessive. Do NOT measure areas that do not make contact. Such areas  include  those  between  the  connecting  rod  and piston bosses and areas under the oil holes and grooves. You can press bushings out of the rod with a mandrel and  an  arbor  press  or  with  special  tools,  as  shown  in figure  3-26.  You  can  also  remove  bushings  by  firt shrinking them with dry ice. Dry ice will also make it easier to insert the new bushing. Figure 3-25.—Measuring a piston pin and piston bushing for wear. 3-17