LO 7-9 Outline diagnostic techniques used for evaluating the condition of a piston ring-to-cylinder wall seal

Cylinder Component Service

Cylinder component servicing consists of testing, removing, disassembling, inspecting, and replacing pistons, piston rings, and connecting rods. A worn engine may be indicated when oil consumption is increased, engine power is reduced, fuel consumption is increased, or the engine runs rough, misfires, produces excessive exhaust emissions or has excessive crankcase blowby. While time and distance are the typical wear factors, there are many causes for premature wear and failure of cylinder components, including problems with oil quality, the air induction system leaking dirt into the engine, internal coolant and fuel leaks, or failures in the EGR system that allow excess recirculation of exhaust gas into the cylinders. Inspecting and diagnosing worn or damaged cylinder components should begin with evaluating the cylinder wear and ring-to-cylinder wall sealing. This can be performed non-intrusively, without disassembling the engine. Inspection of cylinder components can also take place after disassembly.

Most cylinder wear takes place at the top of the cylinder and tapers toward the bottom Figure 58. Lack of lubrication oil above the oil control ring and intermediate ring is the primary reason for most wear taking place in the ring turnaround area. This means cylinder wear is usually tapered, with the widest dimension of the cylinder near the top in a worn-out engine. Major side thrust force is responsible for wear in the middle of the cylinder wall where the angle between the connecting rod and the piston, combined with high cylinder pressures during the power stroke, force the piston against the cylinder wall Figure 59.

FIGURE 07-58: A view of a cylinder from the crankcase. Most wear takes place at the top and tapers toward the bottom. Major side thrust force causes wear in the middle of the cylinder wall.

FIGURE 07-59: Cylinders wear in a tapered shape, with the top of the cylinder wearing away fastest.