FAA Aviation Mechanic Powerplant (AMP)Engine InspectionEasy
A technician is checking valve clearance on a radial engine cylinder equipped with solid lifters (no hydraulic units). To obtain an accurate feeler gauge reading, the piston in that cylinder should be positioned at:
- ABottom dead center on the compression stroke
- BTop dead center on the exhaust stroke, with the exhaust valve open
- CMid-stroke on the intake stroke
- DTop dead center on the compression stroke, with both valves closed
Show answer & explanationAnswer & explanation
Correct answer: D. Top dead center on the compression stroke, with both valves closed
Valve clearance on solid-lifter (mechanical) valve trains must be checked when the piston is at TDC on the compression stroke, because both valves are fully closed and the cam follower rests on the base circle of the cam, giving the true clearance value.
Why the other options are wrong
- A. BDC compression does not correspond to the closed-valve, base-circle cam position.
- B. At TDC exhaust the intake valve may be opening (valve overlap), giving a false reading.
- C. Mid-stroke on intake means a valve is open, making clearance measurement inaccurate.
Mechanical Valve Clearance Check
On engines with solid lifters, valve clearance is measured with a feeler gauge only when the piston is at TDC compression, ensuring both valves are closed and the cam follower is on the base circle.
- Applies to radial and some opposed engines with solid lifters
- TDC compression = both valves closed
- Feeler gauge measures rocker-to-valve stem gap
- Out-of-limit clearance affects valve timing and engine performance
Memory trick: 'Compression Closes the Case' — TDC compression closes both valves for a true reading.