ASE A1 Engine RepairEngine Block AssemblyMedium
A technician is performing a final check on a newly assembled engine before starting it. They notice that the piston for cylinder #3 does not move smoothly through its bore when rotated by hand, feeling tighter than the other pistons. All connecting rod caps are torqued correctly. What is the MOST likely cause of this issue?
- AAn out-of-round or tapered cylinder bore for cylinder #3.
- BAn undersized piston installed in cylinder #3.
- CExcessive crankshaft end play.
- DIncorrectly installed piston rings on cylinder #3.
Show answer & explanationAnswer & explanation
Correct answer: A. An out-of-round or tapered cylinder bore for cylinder #3.
If a specific piston feels tighter and does not move smoothly, and the connecting rod caps are correctly torqued, the most likely cause is an issue with the cylinder bore itself, such as being out-of-round, tapered, or having insufficient piston-to-cylinder wall clearance. This would create binding as the piston travels through the bore.
Why the other options are wrong
- B. An undersized piston would result in excessive clearance and a 'loose' feeling, not a tight one.
- C. Excessive crankshaft end play would cause axial movement of the crankshaft, not binding of an individual piston within its bore.
- D. Incorrectly installed piston rings (e.g., misaligned gaps, broken rings) could cause binding, but an out-of-round or tapered bore is a more fundamental reason for overall tightness.
Cylinder Bore Geometry Issues
Cylinder bore geometry issues, such as out-of-roundness or taper, cause uneven piston-to-cylinder wall clearance, leading to binding, excessive wear, or poor ring sealing.
- Out-of-round means the bore is not perfectly circular.
- Taper means the bore diameter changes from top to bottom.
- Both require machining (boring and honing) to correct.
Memory trick: Bore 'B'inding means 'B'ad shape.