FAA Aviation Mechanic Powerplant (AMP)Engine InspectionMedium
During overhaul, a technician measures the intake valve guide bore of a cylinder using a telescoping gauge and micrometer. The measured diameter exceeds the manufacturer's maximum service limit. What is the most likely operational consequence of continuing to use this cylinder without repair?
- AReduced valve lift causing incomplete combustion
- BReduced valve spring tension causing valve float at high RPM
- CIncreased valve seat interference causing the valve to stick open
- DExcessive valve stem-to-guide clearance causing oil to be drawn into the combustion chamber
Show answer & explanationAnswer & explanation
Correct answer: D. Excessive valve stem-to-guide clearance causing oil to be drawn into the combustion chamber
A worn valve guide bore increases stem-to-guide clearance, allowing the valve to rock side to side. On intake valves this excess clearance commonly draws oil past the guide into the combustion chamber, increasing oil consumption and causing spark plug fouling. This is unrelated to spring tension, seat interference, or cam lift.
Why the other options are wrong
- A. Valve lift is controlled by the cam and pushrod, not the guide bore.
- B. Valve guide wear does not affect spring tension.
- C. Guide wear increases, not decreases, clearance, and does not cause sticking.
Valve Guide Wear
Enlargement of the valve guide bore beyond service limits, measured with a telescoping gauge and micrometer, that allows excessive valve stem side movement.
- Measured with telescoping gauge and outside micrometer
- Excess clearance on intake guides causes oil consumption and fouling
- Excess clearance on exhaust guides can cause valve guttering from poor seating
- Repaired by reaming oversize or replacing/knurling the guide
Memory trick: Loose guide, oily ride — oil sneaks past a wobbly valve stem.