Chapter 3—ENGINE MAINTENANCEother ring and piston troubles. In addition tosymptoms and causes of piston ring troubles,there are other factors that may also be responsi-ble either for low compression or for excessive oilconsumption.When a cylinder with a low compressionpressure is located, the possibility of the cause be-ing some factor other than excessive wear shouldbe eliminated before the pistons rings aredisassembled or replaced. Look at figure 3-18. Ofthe causes listed under “Other factors which maycause low compression pressure” are a, b, c, d,and there are causes that would affect the pressurein only one cylinder assembly of a multicylinderengine. Causes f, g, and h may affect a group ofcylinders, or possibly all cylinders. Therefore,when symptoms indicate compression ring wearconsider first other possibilities. Excessive oil con-sumption is generally associated with worn oilrings, but there are other factors which may causeabnormal oil usage, and these should be checkedbefore replacement of oil rings is undertaken.Oxidation of the lube oil leaves carbondeposits on the rings and in the grooves. It iscaused by excessive operating temperatures. Thecarbon buildup limits movement and expansionof the rings, prevents the rings from following thecylinder contour and sealing the cylinder, and maycause sticking, excessive wear, or breakage.Proper clearance must exist between the ringand land as well as behind the ring, since insuffi-cient ring groove clearance can cause the rings tostick. 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 ringsare likely to become loaded by inertia forces andby side thrust on the piston—forces which shouldbe borne solely by the skirt of trunk-type pistons.Two factors that cause improper ringclearance are:1. Abnormal amount of carbon deposits onrings and in grooves.2. Improper dimensions. New rings must havethe proper thickness, width, diameter, andgap.One cause of undue loads on a ring could beinsufficient gap clearance. This condition wouldcause the ring to be forced out and into a portof a ported cylinder, and possibly result inbreakage.A bright spot found on each end of a brokenring indicates insufficient gap clearance. Sufficientgap clearance must exist at both the top and thebottom of the cylinder bore when rings areinstalled.Sticking and binding of the ring may resultfrom insufficient ring pressure. The tendency ofthe ring to return to its original shape pushes itagainst the cylinder wall, and makes the initialseal. The pressure of the combustion gases behindthe rings reinforces this seal. Pressures (compres-sion and combination) within the cylinder forcethe combustion rings down and cause a seal be-tween the bottom side of the rings and the upperside of the lands; therefore, properly wearing ringswill appear shiny on the outer face and bottomside. Any discoloration (usually appearing asblack lines) indicates the leakage of gases past therings. Extended use and overheating may weakenrings to the point where they do not seatproperly, and the rings are then likely to bind inthe grooves. A check of the free gap for a pistonring will indicate the ring’s condition with respectto sealing qualities. If the instruction manual doesnot give a prescribed dimension for free gap, com-pare the gap with that of a new ring.Conditions which cause piston rings to stickin the grooves, wear excessively, or break are oftenthe result of using improper lube oil. Some lubeoils cause a resinous gumlike deposit to form onengine parts. Trouble of this nature can beavoided by using Navy-approved oils, or oilrecommended by the manufacturer.Probably the greatest factor affecting thewearing of piston rings is a worn cylinder liner.Therefore, when new rings are installed, surfacecondition, amount of taper, and out-of-roundnessof the liner must all be considered. The ring is inthe best position to make allowance for cylinderwear if the ring gaps are in line with the pistonbosses. Gaps of adjacent rings should be staggered180° to reduce gas leakage.With the wearing away of material near thetop of a cylinder liner, a ridge will gradually beformed. When a piston is removed, this ridge mustalso be removed, even though it has caused nodamage to the old set of rings. The new rings willtravel higher in the bore by an amount equal tothe wear of the old rings, and the replacement ofthe connecting rod bearing inserts will also in-crease piston travel. As the top piston ring willstrike the ridge because of this increase in travel,3-29
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