Some engines have ports through which pistons can be  inspected.  If  inspection  reveals  that  the  piston  is defective,  remove  the  piston  assembly.  Figure  3-37 illustrates  testing  for  stuck  piston  rings  through  the scavenging-air   port. If the condition of an engine without cylinder ports indicates that a piston inspection is required, you must take the whole piston assembly out of the cylinder. Engine  bearings  must  be  carefully  fitted  or  installed according to the manufacturer’s instructions. When an engine cannot be jacked over because of an improperly fitted  bearing,  someone  probably  failed  to  follow instructions when the unit was being reassembled. Engine Cannot Be Cranked but Can Be Barred Over You can trace most of the troubles that prevent an engine from cranking, but not serious enough to prevent barring  over,  to  the  starting  system.  Although  other factors  may  prevent  an  engine  from  cranking,  only troubles  related  to  starting  systems  are  identified  in  this chapter. If an engine fails to crank when you apply starting power, first check the turning or jacking gear to be sure it  is  disengaged.  If  this  gear  is  not  the  source  of  the trouble, the trouble is probably with the starting system. Figure 3-37.—Checking the condition of the piston rings. Engine Can Be Cranked, but Fails to Start Although  the  design  of  air  starting  systems  may vary,  the  function  remains  the  same.  In  general,  such systems   must   have   a   source   of   air,   such   as   the compressor or the ship’s air system; a storage tank; air flask(s); an air timing mechanism; and a valve in the engine cylinder to admit the air during starting and to seal the cylinder while the engine is running. All  air  starting  systems  have  a  unit  that  admits starting air to the proper cylinder at the proper time. The type  of  unit  as  well  as  its  name-timer,  distributor,  air starting   pilot   valve,   air   starting   distributor,   or   air distributor-may vary from one system to another. The types   of   air   timing   mechanisms   are   the   direct mechanical   lift,   the   rotary   distributor,   and   the plunger-type distributor valve. The timing mechanism of  an  air  starting  system  is  relatively  trouble-free  except as  noted  in  the  following  situations. DIRECT   MECHANICAL   LIFT.—The    direct mechanical  lift  air  timing  mechanism  includes  cams, pushrods, and rocker arms. These parts are subject to the same  failures  as  engine  cams,  pushrods,  and  rocker arms. Therefore, you can find the causes of trouble in the  actuating  gear  and  the  necessary  maintenance procedures under information covering similar engine parts. Most troubles are a result of improper adjustment. Generally, this involves the lift of the starting air cam or the timing of the air starting valve. The starting air cam must lift the air starting valve enough to give a proper clearance between the cam and the cam valve follower when  the  engine  is  running.  If  there  is  not  enough clearance between these two parts, hot gases will flow between the valve and the valve seat, overheating them. Since the starting air cam regulates the opening of the air starting valve, check those with adjustable cam lobes frequently to ensure that the adjusting screws are tight. Obtain the proper values for lift, tappet clearance, and time of valve opening for a direct mechanical lift timing  mechanism  from  the  manufacturer’s  technical manual for the particular engine. Make adjustments only as  specified. ROTARY   DISTRIBUTOR.—The rotary distributor  timing  mechanism  requires  a  minimum  of maintenance,  but  there  may  be  times  when  the  unit becomes inoperative and you will need to disassemble and inspect it. Generally, the difficulty is caused by a scored rotor, a broken spring, or improper timing. 3-25