Definition
The lower portion of a piston, below the ring grooves, that rides against the cylinder wall and keeps the piston aligned as it travels up and down. The skirt absorbs the side loads created when the connecting rod pushes the piston at an angle during the power and compression strokes.
Plain English
The bottom part of a piston that slides along the inside of the cylinder wall and keeps the piston straight as it moves.
Context Anchor
Seen in aircraft piston-engine construction, maintenance descriptions, engine teardown reports, and discussions of piston or cylinder wear.
Derivation
The word skirt comes from the Old Norse skyrta, meaning a shirt or lower garment. In mechanical use, a skirt is the lower section of a part that hangs below the main body — just like a garment hanging below the waist. The piston skirt is literally the lower portion of the piston that extends below the rings.
Why Pilots Care
Scoring, excessive wear, or damage on the skirt can cause loss of compression, increased oil consumption, and eventual engine failure.
Intuition Check
“Skirt” does not mean a separate cover or piece of fabric here. It means the lower side wall of the piston that helps guide it inside the cylinder.
Example Sentence 1
During the engine teardown, the mechanic noticed scoring on the piston skirt and recommended replacing the piston.
Example Sentence 2
Excessive clearance at the piston skirt produced audible slap on engine start.