Chapter 3ENGINE MAINTENANCE
other ring and piston troubles. In addition to
symptoms and causes of piston ring troubles,
there are other factors that may also be responsi-
ble either for low compression or for excessive oil
consumption.
When a cylinder with a low compression
pressure is located, the possibility of the cause be-
ing some factor other than excessive wear should
be eliminated before the pistons rings are
disassembled or replaced. Look at figure 3-18. Of
the causes listed under Other factors which may
cause low compression pressure are a, b, c, d,
and there are causes that would affect the pressure
in only one cylinder assembly of a multicylinder
engine. Causes f, g, and h may affect a group of
cylinders, or possibly all cylinders. Therefore,
when symptoms indicate compression ring wear
consider first other possibilities. Excessive oil con-
sumption is generally associated with worn oil
rings, but there are other factors which may cause
abnormal oil usage, and these should be checked
before replacement of oil rings is undertaken.
Oxidation of the lube oil leaves carbon
deposits on the rings and in the grooves. It is
caused by excessive operating temperatures. The
carbon buildup limits movement and expansion
of the rings, prevents the rings from following the
cylinder contour and sealing the cylinder, and may
cause sticking, excessive wear, or breakage.
Proper clearance must exist between the ring
and land as well as behind the ring, since insuffi-
cient ring groove clearance can cause the rings to
stick. It is not the function of the rings to sup-
port or position the piston in the cylinder bore,
but if the proper clearance does not exist, the rings
are likely to become loaded by inertia forces and
by side thrust on the pistonforces which should
be borne solely by the skirt of trunk-type pistons.
Two factors that cause improper ring
clearance are:
1. Abnormal amount of carbon deposits on
rings and in grooves.
2. Improper dimensions. New rings must have
the proper thickness, width, diameter, and
gap.
One cause of undue loads on a ring could be
insufficient gap clearance. This condition would
cause the ring to be forced out and into a port
of a ported cylinder, and possibly result in
breakage.
A bright spot found on each end of a broken
ring indicates insufficient gap clearance. Sufficient
gap clearance must exist at both the top and the
bottom of the cylinder bore when rings are
installed.
Sticking and binding of the ring may result
from insufficient ring pressure. The tendency of
the ring to return to its original shape pushes it
against the cylinder wall, and makes the initial
seal. The pressure of the combustion gases behind
the rings reinforces this seal. Pressures (compres-
sion and combination) within the cylinder force
the combustion rings down and cause a seal be-
tween the bottom side of the rings and the upper
side of the lands; therefore, properly wearing rings
will appear shiny on the outer face and bottom
side. Any discoloration (usually appearing as
black lines) indicates the leakage of gases past the
rings. Extended use and overheating may weaken
rings to the point where they do not seat
properly, and the rings are then likely to bind in
the grooves. A check of the free gap for a piston
ring will indicate the rings condition with respect
to sealing qualities. If the instruction manual does
not give a prescribed dimension for free gap, com-
pare the gap with that of a new ring.
Conditions which cause piston rings to stick
in the grooves, wear excessively, or break are often
the result of using improper lube oil. Some lube
oils cause a resinous gumlike deposit to form on
engine parts. Trouble of this nature can be
avoided by using Navy-approved oils, or oil
recommended by the manufacturer.
Probably the greatest factor affecting the
wearing of piston rings is a worn cylinder liner.
Therefore, when new rings are installed, surface
condition, amount of taper, and out-of-roundness
of the liner must all be considered. The ring is in
the best position to make allowance for cylinder
wear if the ring gaps are in line with the piston
bosses. Gaps of adjacent rings should be staggered
180° to reduce gas leakage.
With the wearing away of material near the
top of a cylinder liner, a ridge will gradually be
formed. When a piston is removed, this ridge must
also be removed, even though it has caused no
damage to the old set of rings. The new rings will
travel higher in the bore by an amount equal to
the wear of the old rings, and the replacement of
the connecting rod bearing inserts will also in-
crease piston travel. As the top piston ring will
strike the ridge because of this increase in travel,
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