Chapter 3—ENGINE MAINTENANCEthe cylinder. This allows the crown and ring landsto drag on the cylinder wall. The results of drag-ging can be determined by visually inspecting theparts of the piston in question. However, mostof the pistons now in use in the Navy are free fromthis trouble, since the crown and ring lands areof smaller diameter than the skirt and do not con-tact the cylinder wall.Some piston wear is normal in any engine; theamount and rate depends on several controllablefactors. The causes of excessive piston wear arealso the causes of other piston troubles.One of the factors controlling wear is lubrica-tion. An adequate supply of oil is essential to pro-vide the film necessary to cushion the piston andother parts within the cylinder and prevent metal-to-metal contact. Inadequate lubrication will notonly cause piston wear but the extra friction mayalso cause piston seizure, land breakage, andpiston pin bushing wear.Lack of lubrication is caused either by a lackof lube oil pressure or by restricted oil passages.The pressure-recording instruments usually givewarning of low oil pressure before any great harmoccurs. However, clogged passages offer no suchwarnings. Only by inspecting and cleaning thepiston and connecting rod assembly may youinsure adequate lubrication.Another controllable factor that may bedirectly or indirectly responsible for many pistontroubles is improper cooling water temperatures.If an engine is operated at higher than thespecified temperature limits, lubrication troubleswill develop. High cylinder surface temperatureswill reduce the viscosity of the oil. As the cylinderlubricant thins, it will run off the surfaces. Theresulting lack of lubrication leads to excessivepiston and liner wear. On the other hand, if theengine is operated at temperatures that are belowthose specified, viscosity will be increased, andthe oil will not readily reach the parts requiringlubrication.Oil plays an important part in the cooling ofthe piston crown. If the oil flow to the undersideof the crown is restricted, deposits caused byoxidation of the oil will accumulate and lower therate of heat transfer. For this reason, the under-side of each piston crown should be thoroughlycleaned whenever pistons are removed.While insufficient lubrication and unevencooling may cause ring land failure, excessive oiltemperatures may cause piston seizure. Anincrease in the rate of oxidation of the oil mayresult in clogged oil passages or damage to pistonpin bushings.Seizure and excessive wear of pistons may becaused by improper fit. New pistons or liners mustbe installed with the piston-to-cylinder clearancesspecified in the manufacturer’s technical manual.If clearance is insufficient, a piston will NOT wearin and will probably bind. The resulting excesssurface temperatures may lead to seizure orbreakage.Binding increases wear and shortens piston lifeby scuffing the liner or galling the piston skirt.Scuffing roughens the liner so that an abrasiveaction takes place on the piston skirt, thusgenerating additional heat which may distort orcrack the piston or liner. Galling, especially onaluminum pistons, causes the metal to be wipedin such a manner that the rings bind in thegrooves.A loose fitting piston may be just as destruc-tive as one which is too tight. A loose piston maycause dragging and cocking of the piston, whichin turn may cause broken or cracked ring groovelands.Excessive wear on the piston and piston pinbushing may be caused by either an overload orby an unbalanced load. Overloading an engineincreases the forces on the pistons and subjectsthem to higher temperatures, thus increasing theirrate of wear. There should be a load balance onall pistons at all times. Balance of an engine isdetermined by checking the exhaust gastemperature at each cylinder, the rack settings,and the firing and compression pressures.Cracking of the lands of a piston is caused byinsufficient ring groove clearance. For correctpiston ring operation, proper clearance must bemaintained between the ring and the land, andalso between the ends of the ring. This is necessaryin order that the ring may be free to flex at alltemperatures of operation. The clearance dependsupon the ring and the materials involved.After installing a ring, check the clearance be-tween the ring and the land. This check is made3-27
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