Piston assemblies may have the trunk-type or the crosshead-type pistons. The majority of engines in use by the Navy have trunk-type pistons. Since the troubles encountered with crosshead pistons are very similar to those en-countered with the trunk type, only the latter is discussed here.

PISTONS

Trunk-type pistons are subject to such forces as gas pressure, side thrust, inertia, and friction. These forces, together with overheating and the presence of foreign matter, may cause such troubles as piston wear, cracks, piston seizure, and piston pin bushing wear (see figure 3-17). Piston wear is characterized by an excessive clearance between the piston and the cylinder. Symptoms of excessive clearance between a piston and cylinder are piston slap and excessive oil con-sumption. Piston slap occurs just after top dead center and bottom dead center, as the piston shifts its thrust from one side to the other. As the cylinder taper increases with wear, oil consump-tion increases. Since taper causes the rings to flex on each stroke of the piston, excessive ring wear

Figure 3-17.-Piston troubles and their causes.

occurs, allowing lubricating oil to pass and be burned in the cylinder. This results in the accumulation of excessive carbon deposits on the piston, the combustion chamber, and the engine exhaust valves or ports. This accumulation of car-bon deposits will cause erratic operation and greatly reduce engine efficiency.

Occasionally pistons and liners become suffi-ciently worn to permit the piston to cock over in the cylinder. This allows the crown and ring lands to drag on the cylinder wall. The results of drag-ging can be determined by visually inspecting the parts of the piston in question. However, most of the pistons now in use in the Navy are free from this trouble, since the crown and ring lands are of smaller diameter than the skirt and do not con-tact the cylinder wall.

Some piston wear is normal in any engine; the amount and rate depends on several controllable factors. The causes of excessive piston wear are also 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 and other parts within the cylinder and prevent metal-to- metal contact. Inadequate lubrication will not only cause piston wear but the extra friction may also cause piston seizure, land breakage, and piston pin bushing wear.

Lack of lubrication is caused either by a lack of lube oil pressure or by restricted oil passages. The pressure-recording instruments usually give warning of low oil pressure before any great harm occurs. However, clogged passages offer no such warnings. Only by inspecting and cleaning the piston and connecting rod assembly may you insure adequate lubrication.

Another controllable factor that may be directly or indirectly responsible for many piston troubles is improper cooling water temperatures. If an engine is operated at higher than the specified temperature limits, lubrication troubles will develop. High cylinder surface temperatures will reduce the viscosity of the oil. As the cylinder lubricant thins, it will run off the surfaces. The resulting lack of lubrication leads to excessive piston and liner wear. On the other hand, if the engine is operated at temperatures that are below those specified, viscosity will be increased, and the oil will not readily reach the parts requiring lubrication.

Oil plays an important part in the cooling of the piston crown. If the oil flow to the underside of the crown is restricted, deposits caused by oxidation of the oil will accumulate and lower the rate of heat transfer. For this reason, the under-side of each piston crown should be thoroughly cleaned whenever pistons are removed. While insufficient lubrication and uneven cooling may cause ring land failure, excessive oil temperatures may cause piston seizure. An increase in the rate of oxidation of the oil may result in clogged oil passages or damage to piston pin bushings.

Seizure and excessive wear of pistons may be caused by improper fit. New pistons or liners must be installed with the piston-to-cylinder clearances specified in the manufacturers technical manual. If clearance is insufficient, a piston will NOT wear in and will probably bind. The resulting excess surface temperatures may lead to seizure or breakage.

Binding increases wear and shortens piston life by scuffing the liner or galling the piston skirt. Scuffing roughens the liner so that an abrasive action takes place on the piston skirt, thus generating additional heat which may distort or crack the piston or liner. Galling, especially on aluminum pistons, causes the metal to be wiped in such a manner that the rings bind in the grooves.

A loose fitting piston may be just as destruc-tive as one which is too tight. A loose piston may cause dragging and cocking of the piston, which in turn may cause broken or cracked ring groove lands.

Excessive wear on the piston and piston pin bushing may be caused by either an overload or by an unbalanced load. Overloading an engine increases the forces on the pistons and subjects them to higher temperatures, thus increasing their rate of wear. There should be a load balance on all pistons at all times. Balance of an engine is determined by checking the exhaust gas temperature at each cylinder, the rack settings, and the firing and compression pressures. Cracking of the lands of a piston is caused by insufficient ring groove clearance. For correct piston ring operation, proper clearance must be maintained between the ring and the land, and also between the ends of the ring. This is necessary in order that the ring may be free to flex at all temperatures of operation. The clearance depends upon the ring and the materials involved. After installing a ring, check the clearance be-tween the ring and the land. This check is made

Figure 3-18.Piston ring troubles, their symptoms and causes.

with a thickness gage, and must be made com-pletely around the piston.

Replace most damaged or excessively worn pistons. Since replacement of damaged pistons is usually necessary, shipboard repair parts should always be maintained at full allowance.