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

1.3. Piston Service

Piston Service

A piston has a micro-finish on the skirt that allows it to retain oil, which improves wear characteristics. It is important to remember that this finish should not be damaged during servicing. Do not clean pistons with abrasive cleaners, wire wheels, or other items that can damage the finish of the piston skirt. The best method for cleaning aluminum pistons is to soak them in a mild aluminum-compatible cleaner followed by steam cleaning. Hard carbon deposits on the crown may be scraped away with a plastic scraper.

Visual inspection of pistons should be made to find evidence of cracking, overheating, and wear Figure 60. The presence of vertical or horizontal cracks in the piston bowl or piston pin bore gives important clues when performing failure analysis. Horizontal cracks usually indicate a bonding problem between the piston skirt and the crown. Vertical cracks in the bowl often indicate high cylinder temperatures. Cracking between the crown and the piston pin bore indicate excessive cylinder pressure Figure 61.

FIGURE 07-60: Inspection points for pistons.

FIGURE 07-61: Piston cracks between the piston pin bore and the crown indicate abnormally sharp spikes in cylinder pressure caused by conditions such as the improper use of ether as a starting aid or hydrostatic locks.

Worn ring grooves will cause excessive oil consumption. Wear in the piston’s ring grooves is typically checked with specialized OEM gauge pins Figure 62. Specialized OEM gauge pins fit snuggly into the compression piston ring groove. A pair of springs holds two gauge pins in the grooves, and the distance between the gauge pins is measured with a micrometer. If the piston ring groove wear is excessive, the pins will sit farther into the groove and the dimension between the pins will become smaller. Comparing actual, measured dimensions with manufacturer specifications will determine whether the piston can be reused. Another way to measure wear in the piston’s ring grooves is with a go-no-go gauge Figure 63. To measure piston ring grooves and cylinder wear, follow the guidelines in Skill Drill 7-1.

FIGURE 07-62: Measuring piston ring groove wear with gauge pins and a micrometer.

FIGURE 07-63: Ring groove wear can be checked with a go-no-go gauge. If the gauge touches the piston, the ring groove is excessively worn.

The clearances between a piston and a cylinder wall are best measured with a long feeler blade Figure 64. This is done by placing the piston in the cylinder. The thickest possible feeler blade is then slid in between the piston and the cylinder wall 90 degrees to the wrist pin. The blade should not be forced into the clearance, but a 5–8 lb (2.3–3.6 kg) drag should be felt when pulling the blade out of the clearance. Generally, clearances of between 0.003–0.006" (0.0076–0.152 mm) are encountered in cylinders that fit well. Less clearance than this prevents the piston from properly expanding while more clearance will create noise, cause high oil consumption, and indicates excessive wear or damage to the piston and cylinder wall Figure 65. To measure the clearance between the piston and the cylinder wall, follow the guidelines in Skill Drill 7-2.

FIGURE 07-64: Measuring piston-to-cylinder wall clearance is performed using a long feeler blade between the piston and the cylinder wall.

FIGURE 07-65: Wear points on a worn piston. Worn pistons tilt and scrape oil up into the combustion chamber.