LO 8-13 Outline the scope of cylinder block service and describe common service operations

1.9. Maintenance of Cylinder Walls and Bearings

Maintenance of Cylinder Walls and Bearings

Cylinder walls and engine bearings are the two areas that have the highest degree of wear; as a consequence, they are the most significant factors limiting engine service life. For this reason, servicing and maintaining these two parts of an engine are commonly performed during major and minor overhauls. It is important for technicians to understand the factors influencing wear to anticipate features of these areas to inspect. Understanding wear factors helps to form a foundation for service practices.

Cylinder Wall Wear
Diesel engine cylinder walls last longer than their gasoline counterparts for several reasons. First, slower engine speeds reduce the amount of piston travel. Second, diesel fuel acts like a lubricant to minimize wear from friction. Third, cylinder block and liner hardening processes and the use of special alloys further increase the durability of diesel blocks. However, cylinder block walls progressively wear with increasing engine hours and distance traveled. The greatest amount of wear occurs near the uppermost part of the compression ring travel path and diminishes down the cylinder to the bottom Figure 94. This gives a worn cylinder a bell-shaped wear pattern observed at the top of the cylinder sleeve. The reasons for the greatest amount of wear near the top of the cylinder wall causing the cylinder wall to taper include the following:

• Higher cylinder pressures near the top part of the piston’s travel path increase ring pressure against the cylinder walls because gas pressure behind the rings forces the piston rings outward from their grooves.
• Higher cylinder temperatures make the cylinder wall softer and more prone to wear.
• Lack of upper cylinder wall lubricant because lubrication oil can only be carried into this area on the face of the compression ring.

Long-stroke engines will have more wear mid-stroke due to a higher amount of major thrust. The side thrust is produced by the angle of the connecting rod with the piston, which tends to push the piston sideways into the cylinder wall when cylinder pressures are high. The angle formed between the connecting rod and the piston forces the piston against the cylinder wall during the compression and power strokes. The force is greatest during the power stroke, producing a major thrust force and a minor thrust force during the compression stroke. The longer stroke distance of the diesel makes for higher thrust angle forces Figure 95. In heavily loaded engines, wear is greatest at mid-stroke, when piston velocity and connecting rod angles are highest. To measure cylinder wall wear, follow the steps in Skill Drill 8-3.

To visually evaluate cylinder wear, follow the guidelines in Skill Drill 8-4.

FIGURE 08-94: Cylinder wall wear is tapered and greatest in the upper third of the cylinder. The lack of upper cylinder wall lubrication combined with higher cylinder wall temperatures and ring pressure produce the greatest cylinder wear in this region.

FIGURE 08-95: Cylinder wear at mid-stroke is a common condition in engines with high thrust forces.