FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesEasy
A technician is inspecting a wet-type piston pin (piston pin bushing) removed from a reciprocating engine cylinder assembly. The pin is free to rotate in both the piston bosses and the connecting rod bushing. This describes which type of piston pin retention?
- AInterference-fit piston pin
- BFull-floating piston pin
- CSemi-floating piston pin
- DFixed piston pin
Show answer & explanationAnswer & explanation
Correct answer: B. Full-floating piston pin
A full-floating piston pin is free to rotate in both the piston pin bosses and the connecting rod small-end bushing, distributing wear evenly. It is retained axially by plugs, snap rings, or spring clips to prevent it from contacting the cylinder wall.
Why the other options are wrong
- A. An interference-fit pin is pressed tightly into one part, which is the opposite of a floating design.
- C. A semi-floating pin is fixed (pressed) in one location, usually the rod, and rotates only in the other.
- D. A fixed pin does not rotate at all and is not used in modern aircraft engines.
Full-Floating Piston Pin
A piston pin free to rotate in both the piston bosses and the connecting rod bushing, retained axially by plugs or retaining rings.
- Rotates freely in both piston and rod
- Retained by plugs, snap rings, or spring clips
- Distributes wear evenly around the pin circumference
Memory trick: Full-floating pin floats free in BOTH holes, held by end plugs.