FAA Aviation Mechanic Powerplant (AMP)Engine InspectionEasy
A technician installs new piston rings in a reciprocating engine cylinder and checks ring end gap by squaring the ring in the bore with an inverted piston, then measuring the gap with a feeler gauge. The measured gap is 0.006 inch, but the overhaul manual specifies a minimum gap of 0.010 inch. What should the technician do?
- ASoak the ring in oil to allow it to seat and close the gap over time
- BFile the ring ends to slightly increase the end gap to meet the minimum specification
- CInstall the ring as is since the gap is close enough to the minimum
- DInstall the ring rotated 90 degrees from the original measurement position
Show answer & explanationAnswer & explanation
Correct answer: B. File the ring ends to slightly increase the end gap to meet the minimum specification
Insufficient ring end gap can cause the ring ends to butt together as the ring expands from heat during operation, leading to scuffing, scoring, or a stuck/broken ring. The correct correction is to carefully file the ring ends to open the gap to at least the minimum specified value.
Why the other options are wrong
- A. Soaking in oil has no effect on the mechanical dimension of the ring gap.
- C. Installing an undersized gap risks ring butting and cylinder scoring at operating temperature.
- D. Rotating the ring does not change its actual end gap dimension.
Piston Ring End Gap
The clearance between the ends of a piston ring when installed squarely in the cylinder bore, required to allow for thermal expansion during engine operation.
- Measured with a feeler gauge while ring is squared using an inverted piston
- Insufficient gap causes ring butting, scuffing, or seizure when hot
- Excessive gap causes blow-by and loss of compression
- Corrected by carefully filing the ring ends if gap is too small
Memory trick: Too tight, rings fight (butt) — file for space to expand.