Some engines have ports through which pistons canbe inspected. If inspection reveals that the piston isdefective, remove the piston assembly. Figure 3-37illustrates testing for stuck piston rings through thescavenging-air port.If the condition of an engine without cylinder portsindicates that a piston inspection is required, you musttake the whole piston assembly out of the cylinder.Engine bearings must be carefully fitted or installedaccording to the manufacturer’s instructions. When anengine cannot be jacked over because of an improperlyfitted bearing, someone probably failed to followinstructions when the unit was being reassembled.Engine Cannot Be Cranked but Can Be BarredOverYou can trace most of the troubles that prevent anengine from cranking, but not serious enough to preventbarring over, to the starting system. Although otherfactors may prevent an engine from cranking, onlytroubles related to starting systems are identified in thischapter.If an engine fails to crank when you apply startingpower, first check the turning or jacking gear to be sureit is disengaged. If this gear is not the source of thetrouble, 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 StartAlthough the design of air starting systems mayvary, the function remains the same. In general, suchsystems must have a source of air, such as thecompressor or the ship’s air system; a storage tank; airflask(s); an air timing mechanism; and a valve in theengine cylinder to admit the air during starting and toseal the cylinder while the engine is running.All air starting systems have a unit that admitsstarting air to the proper cylinder at the proper time. Thetype of unit as well as its name-timer, distributor, airstarting pilot valve, air starting distributor, or airdistributor-may vary from one system to another. Thetypes of air timing mechanisms are the directmechanical lift, the rotary distributor, and theplunger-type distributor valve. The timing mechanismof an air starting system is relatively trouble-free exceptas noted in the following situations.DIRECT MECHANICAL LIFT.—The directmechanical lift air timing mechanism includes cams,pushrods, and rocker arms. These parts are subject to thesame failures as engine cams, pushrods, and rockerarms. Therefore, you can find the causes of trouble inthe actuating gear and the necessary maintenanceprocedures under information covering similar engineparts.Most troubles are a result of improper adjustment.Generally, this involves the lift of the starting air cam orthe timing of the air starting valve. The starting air cammust lift the air starting valve enough to give a properclearance between the cam and the cam valve followerwhen the engine is running. If there is not enoughclearance between these two parts, hot gases will flowbetween the valve and the valve seat, overheating them.Since the starting air cam regulates the opening of theair starting valve, check those with adjustable cam lobesfrequently 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 lifttiming mechanism from the manufacturer’s technicalmanual for the particular engine. Make adjustments onlyas specified.ROTARY DISTRIBUTOR.—The rotarydistributor timing mechanism requires a minimum ofmaintenance, but there may be times when the unitbecomes inoperative and you will need to disassembleand inspect it. Generally, the difficulty is caused by ascored rotor, a broken spring, or improper timing.3-25
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