Piston Skirt
The lower section of a piston that guides it in a straight line and prevents it from rocking or tilting inside the cylinder.
The piston skirt is the lower portion of the piston body below the ring lands. Its primary function is to guide the piston in a straight path as it travels up and down the cylinder bore, preventing it from rocking or tilting under the angular thrust forces of the connecting rod. The skirt is often coated with low-friction materials (like Teflon or graphite) to reduce wear.
Because the piston experiences lateral thrust forces during the combustion stroke, the piston skirt is pressed hard against the cylinder wall. If the engine suffers from poor lubrication, overheating, or fuel dilution of the oil, the skirt can experience metal-on-metal contact, leading to scuffing and scoring of both the piston and the cylinder wall. This wear increases piston-to-wall clearance, causing a rocking motion known as "piston slap."
Symptoms of piston skirt wear include a metallic rattling noise when the engine is cold (piston slap), high oil consumption, and blue exhaust smoke. Mechanics diagnose skirt damage using a borescope inserted through the spark plug hole or by measuring the piston diameter during engine disassembly. Scored piston skirts require piston replacement and cylinder boring.