Chapter  3—ENGINE  MAINTENANCE Readings are generally taken at the four crank positions:  top  dead  center,  inboard,  near  or  at bottom   dead   center,   and   outboard.   In   some engines,  it  is  possible  to  take  readings  at  bottom dead center. In others, the connecting rod may interfere, making it necessary to take the reading as near as possible to bottom dead center without having the gage come in contact with the con- necting  rod.  The  manufacturer’s  technical  manual for the specific engine provides information con- cerning the proper position of the crank when readings are to be taken. When the gage is in its lowest position, the dial will be upside down, necessitating the use of a mirror and flashlight to obtain a reading. Once the indicator has been placed in position for the first deflection reading, do NOT touch the gage  until  all  four  readings  have  been  taken  and recorded. Variations in the readings obtained at the four crank  positions  will  indicate  distortion  of  the crank. Distortion may be caused by several fac- tors, such as a bent crankshaft, worn bearings, or  improper  engine  alignment.  The  maximum allowable deflection can be obtained from the manufacturer’s technical manual. If the deflec- tion exceeds the specified limit, take steps to deter- mine the cause of the distortion and to correct the trouble. Deflection  readings  are  also  employed  to determine  correct  alignment  between  the  engine and the generator, or between the engine and the coupling. When alignment is being determined, a set of deflection readings is usually taken at the crank nearest to the generator or the coupling. In aligning an engine and generator, it may be necessary  to  install  new  chocks  between  the generator  and  its  base  to  bring  the  deflection within   the   allowable   value.   It   may   also   be necessary to shift the generator horizontally to obtain proper alignment. When an engine and a coupling are to be aligned, the coupling must first be correctly aligned with the drive shaft; then, the engine must be properly aligned to the coupling, rather than the coupling aligned to the engine. CAMSHAFTS In   addition   to   the   camshaft   and   bearing troubles  already  mentioned,  the  cams  of  a camshaft may be damaged as a result of improper valve  tappet  adjustment,  worn  or  stuck  cam followers, or failure of the camshaft gear. Cams are likely to be damaged when a loose valve tappet adjustment or a broken tappet screw causes the valve to jam against the cylinder head, and the push rods to jam against their cams. This will result in scoring or breaking of the cams and followers, as well as severe damage to the piston and the cylinder. Valves must be timed correctly at all times, not only for the proper operation of the engine but also to prevent possible damage to the engine parts. You should inspect frequently the valve actuating  linkage  during  operation  to  determine if it is operating properly. Such inspections should include taking tappet clearances and adjusting, if necessary; checking for broken, chipped, or improperly  seated  valve  springs;  inspecting  push rod end fittings for proper seating; and inspect- ing cam follower surfaces for grooves or scoring. JOURNAL BEARINGS Engine journal bearing failure and their causes may vary to some degree, depending upon the type  of  bearing.  The  following  discussion  of  the causes   of   bearing   failure   applies   to   most bearings—main bearings as well as crank pin bear- ings.  The  most  common  journal  bearing  failures may be due to one or to a combination of the following  causes: 1. Corrosion of bearing materials caused by chemical  action  of  oxidized  lubricating  oils. Oxidation  of  oil  may  be  minimized  by  changing oil at the designated intervals, and by keeping engine temperatures within recommended limits. Bearing failures due to corrosion may be identified by very small pits covering the surfaces. In most instances, corrosion occurs over small bearings areas  in  which  high  localized  pressures  and temperatures exist. Since the small pits caused by corrosion are so closely spaced that they form channels, the oil film is not continuous and the load-carrying area of the bearing is reduced below the point of safe operation. 2.  Surface  pitting  of  bearings  due  to  high localized temperatures that cause the lead to melt. 3-33