FAA Aviation Mechanic Powerplant (AMP)Engine InspectionMedium
During overhaul, a technician measures a reciprocating engine piston pin outside diameter at 0.8740 inch and the piston pin bore inside diameter at 0.8752 inch. The overhaul manual specifies a maximum allowable pin-to-bore clearance of 0.0010 inch. Based on these measurements, what should the technician do?
- AInstall an oversized piston pin to reduce the clearance to zero
- BReject the piston because the measured clearance of 0.0012 inch exceeds the 0.0010 inch limit
- CRework the piston pin bore to reduce clearance without further measurement
- DAccept the piston since the clearance is within the 0.0010 inch limit
Show answer & explanationAnswer & explanation
Correct answer: B. Reject the piston because the measured clearance of 0.0012 inch exceeds the 0.0010 inch limit
Clearance = bore ID − pin OD = 0.8752 − 0.8740 = 0.0012 inch, which exceeds the 0.0010 inch maximum allowed. The piston must be rejected since the wear limit has been exceeded.
Why the other options are wrong
- A. Reducing clearance to zero would create an interference fit, which is improper.
- C. Reworking without confirming against approved data and re-measuring is not proper practice.
- D. Incorrect math — 0.0012 inch exceeds, not meets, the 0.0010 inch limit.
Piston Pin-to-Bore Clearance
The running clearance between a piston pin's outside diameter and the piston pin bore's inside diameter, checked during overhaul against manufacturer limits.
- Clearance = bore ID − pin OD
- Excessive clearance causes pin knock and accelerated wear
- Parts exceeding limits must be rejected, not reworked without approved data
Memory trick: Bore minus pin tells the story — too big a gap, the piston's gone.