A technician measures a reciprocating engine cylinder bore at the top of ring travel and finds a diameter of 5.008 inches, and at the bottom of ring travel finds 5.000 inches. The overhaul manual specifies a standard bore diameter of 5.000 inches with a maximum allowable taper of 0.004 inch. What must the technician conclude?
- AThe cylinder is within limits since the bottom measurement matches standard bore diameter
- BThe cylinder is out-of-round, not tapered, and requires honing only
- CThe cylinder taper of 0.008 inch exceeds the 0.004 inch limit and the cylinder must be reground or replaced
- DThe measurements indicate normal wear and no further action is needed
Show answer & explanationAnswer & explanation
Correct answer: C. The cylinder taper of 0.008 inch exceeds the 0.004 inch limit and the cylinder must be reground or replaced
Cylinder taper is calculated as the difference between the bore diameter at the top and bottom of ring travel: 5.008 − 5.000 = 0.008 inch. This exceeds the manual's maximum allowable taper of 0.004 inch, meaning the cylinder must be reground to an oversize or replaced, since excessive taper causes poor ring seating and increased oil consumption/blow-by.
Why the other options are wrong
- A. Taper is determined by the difference between top and bottom measurements, not just one value.
- B. Out-of-round is measured by comparing diameters at the same height in perpendicular directions, not top-to-bottom.
- D. The calculated taper doubles the allowable limit, so this is not normal acceptable wear.
Cylinder Bore Taper
The difference in cylinder bore diameter measured at the top versus the bottom of piston ring travel, caused by uneven wear, checked against a maximum allowable limit in the overhaul manual.
- Taper = top diameter − bottom diameter of ring travel
- Compared against maximum allowable taper limit in manual
- Excessive taper causes poor ring seating, blow-by, and oil consumption
- Corrected by honing/regrinding to oversize or cylinder replacement
Memory trick: Wide at the top, narrow below — that's taper telling you the bore must go.