FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A mechanic is fitting new piston rings and checks the ring end gap by placing the ring squarely in the cylinder bore and measuring the gap with a feeler gauge. The measured gap is found to be less than the manufacturer's minimum specification. If installed as-is, what is the most likely result during engine operation?
- AExcessive blow-by past the rings, causing low compression readings
- BThe ring ends may butt together as the ring expands from heat, causing scoring or ring breakage
- COil consumption will decrease because the tighter gap improves the oil control seal
- DThe ring will rotate freely in its groove, causing rapid ring groove wear
Show answer & explanationAnswer & explanation
Correct answer: B. The ring ends may butt together as the ring expands from heat, causing scoring or ring breakage
Ring end gap provides clearance for thermal expansion of the ring as the cylinder heats up. If the gap is too small, the ring ends can butt together when heated, generating excessive stress that scores the cylinder wall or breaks the ring. A gap that is too large, by contrast, causes excessive blow-by.
Why the other options are wrong
- A. Excessive blow-by is the result of too LARGE a gap, not too small.
- C. A too-small gap does not improve sealing; it risks mechanical damage before any oil control benefit.
- D. Ring rotation in the groove is unrelated to end gap size.
Piston Ring End Gap
The small clearance measured between the ends of a piston ring when fitted squarely in the cylinder bore, needed to allow for thermal expansion.
- Measured with a feeler gauge with ring squared in bore
- Too small: ring ends butt when hot, causing scoring/breakage
- Too large: excessive blow-by and power loss
Memory trick: Rings need room to grow — too tight and they'll fight each other when hot!