Scored cylinder liners may become scored (scratched) by several means. These scratches degrade the engines performance and require some type of repair.

Scored cylinder liners may be caused by broken piston rings, a defective piston, improper cooling, improper lubrication, or the presence of foreign particles or objects. Dust particles drawn into an engine cylinder will mix with the oil and become an effective but undesirable lapping compound that may cause extensive damage. The importance of keeping the intake air clean cannot be overemphasized.

Another precaution you should take is to make sure that when you replace a cylinder head, you leave no metal chips, nuts, bolts, screws, or tools in the cylinder.

Causes

Scoring may be in the form of deep or shallow scratches in the liner surface. With most liner scoring, there will be corresponding scratches on the piston and piston rings. The symptoms of scoring may be low firing or compression pressure and rapid wear of piston rings. The best method for detecting scoring is visual inspection through liner ports, through the crankcase housing with pistons in their top position, or when the engine is disassembled.

Badly worn pistons and rings may cause scoring because blowby of combustion gases increases the temperature of the liner and may reduce the oil film until metal-to-metal contact takes place. Inspect the pistons and rings carefully. A piston with a rough surface (such as one that has seized) will score the liner.

Scoring as a result of insufficient lubrication or dirt in the lubricating oil can be prevented if lubricating equipment (filters, strainers, and centrifuges) is maintained properly. Lube oil must be purified according to required procedures.

Repairs

Ships force personnel normally do not repair scored liners; they replace them with spare liners. When necessary, liners with minor scoring may be kept in service, if the cause of scoring is eliminated and the minor defects can be corrected. The surface of the liner must be inspected carefully, especially in the region next to the ports, for any burrs, projections, or sharp edges that will interfere with piston and ring travel. Most projections can be removed by handstoning, using a fine stone. Figure 3-4 shows a liner before and after the ports were stoned.