Definition
A reciprocating-engine piston machined so that its skirt is slightly elliptical (oval) when cold rather than perfectly round. The longer diameter is across the axis perpendicular to the piston pin, and the shorter diameter is along the pin axis. As the piston heats up in operation, it expands more along the pin-boss axis (where there is more metal mass), and the skirt becomes essentially round at operating temperature, providing a proper running fit in the cylinder.
Plain English
A piston that is shaped slightly oval when cold so that, once it heats up and the metal expands unevenly, it ends up perfectly round inside the cylinder. This gives a snug, smooth fit at normal operating temperature without binding when cold or being too loose when hot.
Context Anchor
Seen in aircraft engine construction, maintenance, and piston-engine systems discussions.
Derivation
"Cam-ground" refers to the manufacturing method: the piston is machined on a lathe equipped with a cam mechanism that causes the cutting tool to move in and out as the piston rotates, producing the slightly oval (cam-shaped) skirt rather than a true circle.
Why Pilots Care
Ensures proper piston-to-cylinder fit, prevents seizure, and maintains even ring contact for compression and oil control.
Analogy
It is like making a part slightly the “wrong” shape while cold so it becomes the right shape when hot.
Intuition Check
Cam-ground does not mean the piston is worn, damaged, or connected to the engine’s camshaft. It means the piston was intentionally ground to a slightly oval cold shape so heat expansion brings it into proper fit.
Example Sentence 1
Most modern aircraft piston engines use cam-ground pistons so the skirt fits the cylinder wall correctly once the engine reaches operating temperature.
Example Sentence 2
Cam-ground pistons expand evenly inside the cylinder bores once the engine reaches operating temperature.