Cylinder Bore Taper and Oversize Decision
A 4-cylinder 1.6 L gasoline engine is in the machine shop for a rebuild after overheating. The service manual gives a standard bore of 86.000–86.020 mm, maximum taper 0.010 mm, maximum out-of-round 0.008 mm, and piston-to-wall clearance 0.025–0.045 mm measured at 90° to the pin, 12 mm from the top of the skirt. The oversize pistons available are +0.25 mm and +0.50 mm; the +0.25 piston measures 86.235 mm at the specified point. The technician measures cylinder 3 with a calibrated bore gauge at three heights and two axes: top 86.048/86.040 mm, middle 86.035/86.033 mm, bottom 86.028/86.027 mm. Cylinder 2 maximum is 86.031 mm. The shop's boring bar can hold ±0.005 mm, and the hone can remove 0.010–0.015 mm. The customer wants the lowest-cost repair that meets specification and expects the vehicle back in three days.
1. Calculate the taper and out-of-round for cylinder 3. Compare them with the specification and decide whether honing alone can correct the bore.
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Out-of-round is the difference between the two axes at the same height:
- Top: 86.048 − 86.040 = 0.008 mm
- Middle: 86.035 − 86.033 = 0.002 mm
- Bottom: 86.028 − 86.027 = 0.001 mm
Taper is the largest diameter minus the smallest diameter over the full measured height:
- 86.048 − 86.027 = 0.021 mm
The top out-of-round is exactly at the 0.008 mm limit, but the taper is 0.021 mm, which is more than twice the 0.010 mm limit. The bore is also already larger than the standard maximum of 86.020 mm. Honing can remove only 0.010–0.015 mm, so it cannot remove the 0.021 mm taper and restore a straight bore. Honing alone would either leave excessive taper or require a final size that no standard piston can fit. The correct decision is to rebore.
2. Determine whether a +0.25 mm oversize piston will clean up cylinder 3. Show the required final bore, the cleanup allowance, and the resulting clearance.
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The +0.25 piston measures 86.235 mm. Required bore = piston diameter + clearance:
- Minimum bore: 86.235 + 0.025 = 86.260 mm
- Maximum bore: 86.235 + 0.045 = 86.280 mm
A practical target is 86.270 mm, which gives 86.270 − 86.235 = 0.035 mm clearance, inside the 0.025–0.045 mm range. The largest existing diameter in cylinder 3 is 86.048 mm. Cleanup at 86.270 mm is 86.270 − 86.048 = 0.222 mm on diameter, or about 0.111 mm per side. That is well within the boring bar's capability. Cylinder 2, with a maximum of 86.031 mm, also cleans up at the same final size.
Therefore the +0.25 mm piston is the lowest-cost option that meets specification, provided all cylinders are bored to the same final size and each piston is matched to its bore. The +0.50 mm piston is not needed unless inspection after boring reveals deeper damage. The shop should rough-bore to about 86.255–86.260 mm, leave 0.010–0.015 mm for honing, and finish to the target bore.
3. List the measurement and documentation steps needed to verify the finished bore and piston clearance before assembly.
Read the worked answer
Clean the bore and piston, stabilize them at about 20 °C, and zero the bore gauge against a micrometer set to the target size. Measure each bore at three heights and two axes 90° apart, recording every reading. Calculate out-of-round and taper and confirm they are below 0.008 mm and 0.010 mm respectively. Measure each piston at 90° to the pin and 12 mm from the top of the skirt, then subtract the piston diameter from the matching bore to confirm 0.025–0.045 mm clearance. Check the hone pattern and final surface finish; plateau-hone if required by the manual.
Document the final bore size, piston part number and serial or cylinder marking, measured clearance, boring bar setting, hone allowance, and technician initials. Record the before-and-after measurements for each cylinder so the customer and QC inspector can see why the +0.25 mm oversize was selected. Do not mix pistons between bores after final matching.