serrations of either the shaft or the hub have failed for any reason, replace both parts. Obstruction in the Combustion Space Such items as broken valve heads and valve stem locks or keepers that come loose because of a broken valve spring may cause an engine to come to an abrupt stop.   If   an   engine   continues   to   run   when   such obstructions are in the combustion chamber, the piston, liner,  head,  and  injection  nozzle  will  be  severely damaged. Piston Seizure Piston  seizure  may  also  cause  an  engine  to  stop suddenly. The piston becomes galled and scuffed. When this occurs, the piston may possibly break or extensive damage may be done to other major engine parts. The principal  causes  of  piston  seizure  are  insufficient clearance,   excessive   temperatures,   or   inadequate lubrication. Defective Auxiliary Drive Mechanisms Defects in auxiliary drive mechanisms may cause an engine to stop suddenly. Since most troubles in gear trains  or  chain  drives  require  some  disassembly,  this discussion will be limited to the causes of such troubles. Gear failure is the principal trouble in gear trains. Engine  failure  and  extensive  damage  can  occur  because of  a  broken  or  chipped  gear.  If  you  hear  a  metallic clicking  noise  in  the  vicinity  of  a  gear  housing,  it  is almost  a  certain  indication  that  a  gear  tooth  has  broken. Gears  are  most  likely  to  fail  because  of  improper lubrication,    corrosion,    misalignment,    torsional vibration,   excessive   backlash,   wiped   bearings   and bushings, metal   obstructions,   or   improper manufacturing procedures. Gear shafts, bushings and bearings, and gear teeth must be checked during periodic inspections for scoring, wear,  and  pitting.  All  oil  passages,  jets,  and  sprays should  be  cleaned  to  ensure  proper  oil  flow.  All gear-locking  devices  must  fit  tightly  to  prevent longitudinal gear movement. Chains are used in some engines for camshaft and auxiliary  drives;  in  other  engines,  chains  are  used  to drive certain auxiliary rotating parts. Troubles in chain drives are usually caused by wear or breakage. Troubles of this nature may be caused by improper chain tension, lack of lubrication, sheared cotter pins, or misalignment. Figure 3-40 is a summary of the possible troubles that  may  cause  an  engine  to  stall  frequently  or  stop suddenly. There may be some doubt as to the difference between stalling and stopping. In reality, there is none unless   we   associate   certain   troubles   with   each.   In general,  troubles  that  cause  FREQUENT  STALLING are those that can be eliminated with minor adjustments or maintenance. If such troubles are not eliminated, it is quite possible that the engine can be started, only to stall again.  Failure  to  eliminate  some  of  the  troubles  that cause frequent stalling may lead to troubles that cause SUDDEN  STOPPING. ENGINE WILL NOT CARRY A LOAD Many of the troubles that can lead to loss of power in an engine may also cause the engine to stop and stall suddenly or may even prevent it from starting. Compare the list of some of the troubles that may cause a power loss (fig. 3-41) with those in figures 3-36 and 3-40. Such items  as  insufficient  air,  insufficient  fuel,  and  faulty operation  of  the  governor  appear  on  all  three  charts. Many of the troubles listed are closely related, and the elimination of one may eliminate others. The  operator  of  an  internal-combustion  engine  may be  confronted  with  additional  major  difficulties,  such  as those indicated in figure 3-42. Here, again, you can see that many of these possible troubles are similar to those that  have  already  been  discussed  in  connection  with starting failures and with engine stalling and stopping. The discussion that follows covers only those troubles not  previously  considered. ENGINE  OVERSPEEDS When an engine overspeeds, the trouble is usually caused  by  either  the  governor  mechanism  or  the  fuel control linkage, as previously discussed When you need information on a specific fuel system or speed control system, check the manufacturer’s technical manual and the special technical manuals for the particular system. These special manuals are available for the most widely used models of hydraulic governors and overspeed trips, and they contain specific details on testing, adjusting, and repairing each controlling device. ENGINE HUNTS OR WILL NOT STOP Some troubles that may cause an engine to hunt, or vary its rpm at constant throttle setting, are similar to those  that  may  cause  an  engine  to  resist  stopping. Generally, these two forms of irregular engine operation 3-35