FAA Aviation Mechanic Powerplant (AMP)Engine InspectionMedium
During cylinder installation, a mechanic torques the cylinder base nuts to specification but fails to follow the manufacturer's specified torque sequence, tightening them randomly instead of in a star/crossing pattern. What is the primary risk of this practice?
- AExcessive valve clearance after the engine is run
- BUneven clamping load that can distort the cylinder base flange or crankcase mating surface
- CReduced compression ratio due to piston misalignment
- DIncorrect spark plug gap on that cylinder
Show answer & explanationAnswer & explanation
Correct answer: B. Uneven clamping load that can distort the cylinder base flange or crankcase mating surface
Torque sequence matters because tightening fasteners out of the prescribed cross pattern applies uneven clamping forces, which can warp or distort the cylinder base flange and the crankcase split line, potentially causing a poor seal, case fretting, or a future crack; the final torque value alone does not guarantee even clamping.
Why the other options are wrong
- A. Valve clearance is set separately after cylinder installation and is unrelated to base nut sequence.
- C. Compression ratio is fixed by cylinder/piston geometry, not fastener sequence.
- D. Spark plug gap is unrelated to base nut torque sequence.
Cylinder Base Nut Torque Sequence
Cylinder base nuts must be torqued in a specified crossing (star) pattern in incremental steps to ensure even clamping and prevent flange distortion.
- Random tightening causes uneven stress on the base flange
- Sequence typically follows a star/crossing pattern in stages
- Correct final torque value alone does not ensure a proper seal
Memory trick: Tighten like a star, not a scatter, to avoid a warp