The troubles to which piston rings are subject and their symptoms and causes are listed in figure 3-18.

All symptoms and causes shown for ring wear are either directly and indirectly related to 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, breakage of the ring and perhaps of the land is almost certain if the ridge is not removed.