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

1.12. Inspecting for Cylinder Wear

Inspecting for Cylinder Wear

Several methods are used to measure engine wear when an engine is partially or fully disassembled. One effective method to visually evaluate cylinder wear is to examine the ring turnaround area Figure 75. Using a piece of paper to reflect light directly into the crosshatch helps make the cylinder wear more visible. If 20% or more of the crosshatch is missing, 0% service life remains. If 10% of the crosshatch is missing, 50% service life remains. An engine with a 5" (13 cm) stroke and 0.25" (6 mm) of polish has 75% service life remaining.

FIGURE 07-75: You can visually evaluate cylinder wear in the ring turnaround area.

FIGURE 07-76: The ring end gap method is used to check cylinder taper. Any differences when measuring end gap at the top, middle, and bottom of the cylinder identifies dimensional differences.

A second method is to measure the piston ring end gap at three points in the cylinder swept by the piston rings: top, middle, and bottom Figure 76. After squaring a used compression ring in the cylinder, a feeler blade is placed in the end gap of the ring. The maximum clearance in the end gap is measured at three points. Because the circumference of a circle is found by multiplying π (3.14) and the circle’s diameter, the end gap differences are divided by 3.1 (which is rounded from π, which is 3.14). That means a difference of 0.003" (0.076 mm) between the widest and the smallest end gap equals 0.001" (0.025 mm) of taper wear. A general rule for engine wear is that no more than 0.001" (0.025 mm) of taper wear is permissible for every 1" (25 mm) of cylinder bore up to 0.005" (0.127 mm). If wear exceeds this amount, two problems will occur. The first is that the cylinder wall will not be able to retain an oil film for lubricating and sealing the rings because the crosshatch is worn from the cylinder wall. The second is that the rings will open and close too far during each stroke; they will eventually become fatigued from the continual flexing and break.

The dimensions of the cylinder can also be measured using a telescoping gauge set. This is done in three places as well: the top, middle, and bottom of the cylinder, where it is swept by the piston rings. An experienced and skilled technician can measure the bore diameter with a telescoping gauge and an outside micrometer; however, due to the difficulty of developing the skills to accurately use these types of measurement tools, they are seldom used.

The method most commonly used by machinists when reconditioning cylinder bores involves the use of a dial bore gauge, which is a precision measuring tool used to measure taper wear in a cylinder. This tool has a dial-like indicator measuring mechanism. After the gauge is adjusted to a dimension just slightly more than the bore diameter, it is placed in the cylinder and rocked back and forth to sweep the gauge through a perpendicular point or at a right angle to the cylinder wall. The smallest dimension of the bore gauge indicates when the gauge has found the narrowest point in the cylinder. The gauge is moved to several more points in the cylinder to determine what the narrowest dimensions are and then a comparison is made between the measurements. Once again, the maximum permissible variation between the smallest and the largest measurements should be less than 0.005" (0.127 mm), which indicates 0% remaining service life.