Chapter 3ENGINE MAINTENANCE
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
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