FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A technician measures a piston skirt and finds its diameter slightly greater when measured perpendicular to the piston pin than when measured parallel to the pin axis (a cam-ground or elliptical shape). What is the purpose of this design?
- ATo compensate for greater thermal expansion at the piston pin boss area so the piston becomes round when hot
- BTo allow the piston to rock intentionally within the cylinder for better oil distribution
- CTo increase ring seal by making the piston slightly oversized at all points
- DTo reduce piston weight and improve engine balance
Show answer & explanationAnswer & explanation
Correct answer: A. To compensate for greater thermal expansion at the piston pin boss area so the piston becomes round when hot
The piston pin boss area contains more metal mass and heats/expands more than the thinner skirt sections. Manufacturers cam-grind the piston slightly elliptical (smaller across the pin axis) when cold so that, as the engine reaches operating temperature, the pin boss area expands more and the piston becomes essentially round, maintaining proper clearance in the cylinder.
Why the other options are wrong
- B. Intentional rocking would cause excessive wear and noise, not a design goal.
- C. The shape is about differential clearance, not overall oversizing for ring seal.
- D. Weight reduction is unrelated to the elliptical shape; that is achieved by other design features.
Cam-Ground (Elliptical) Piston
A piston skirt machined slightly oval, smaller in diameter along the pin axis than perpendicular to it, so it becomes round when heated due to greater expansion at the pin bosses.
- Pin boss area has more mass and expands more when hot
- Cold piston is intentionally out-of-round
- Piston becomes round at normal operating temperature for proper fit
Memory trick: 'Egg-shaped when cold, round when hot'