Bore measurement, thermal error and a low-cost overhaul decision
During overhaul of a 1.6 L four-cylinder aluminium engine with pressed-in cast-iron liners, a technician measures cylinder 2 with a bore gauge calibrated at 20 °C. Readings at 10 mm, 60 mm and 110 mm from the deck, in the thrust and axial axes, are: top thrust 79.04 mm, top axial 79.02; middle thrust 79.03, middle axial 79.02; bottom thrust 79.01, bottom axial 79.01. The workshop manual gives nominal bore 79.00–79.02 mm, maximum taper 0.05 mm, maximum out-of-round 0.03 mm, and piston-to-bore clearance 0.03–0.05 mm with the original piston marked 78.97 mm. The workshop is at 28 °C and the bore gauge was zeroed on a 79.00 mm setting ring at that temperature; the gauge has a stated accuracy of ±0.005 mm. The customer wants a low-cost repair and asks whether honing with new rings is acceptable. The engine had 160,000 km, overheated once, and has a scuffed piston skirt.
1. Calculate taper, out-of-round and piston-to-bore clearance at the top thrust point. Compare them with the limits and decide whether honing with the original piston is acceptable.
Read the worked answer
Taper is the maximum difference between the largest and smallest bore readings: 79.04 − 79.01 = 0.03 mm, which is within the 0.05 mm limit. Out-of-round at each level is the difference between thrust and axial readings: top 79.04 − 79.02 = 0.02 mm; middle 0.01 mm; bottom 0.00 mm. The maximum 0.02 mm is within the 0.03 mm limit.
Piston-to-bore clearance at the top thrust point using the marked piston is 79.04 − 78.97 = 0.07 mm, against a maximum of 0.05 mm. The top thrust bore is also 0.02 mm above the nominal upper limit of 79.02 mm. Honing removes material and increases the bore, so it cannot reduce a 0.07 mm clearance; it would make the clearance worse. Honing with the original piston and new rings is therefore not acceptable. The correct low-cost route is to measure all cylinders and pistons, then rebore/hone to the next oversize with matching new pistons, or replace/repair the liner, after finding the cause of the scuffed skirt and overheating.
2. Assess the effect of the 28 °C workshop temperature and the gauge accuracy. Could the conclusion change? What additional measurements should be made before authorising a rebore?
Read the worked answer
The readings were taken at 28 °C but the gauge was zeroed on a 79.00 mm setting ring at that same temperature, while the manual dimensions are at 20 °C. Aluminium expands by roughly 23 × 10⁻⁶ per °C, so an 8 °C rise adds about 79 × 23 × 10⁻⁶ × 8 = 0.0145 mm to the block bore. The steel setting ring adds about 79 × 11 × 10⁻⁶ × 8 = 0.007 mm. Zeroing on the expanded ring biases the displayed bore low relative to a true 20 °C ring, so the corrected 20 °C bore is still around 79.03 mm, not smaller enough to bring clearance within 0.05 mm. The gauge accuracy of ±0.005 mm gives a clearance range of roughly 0.065–0.075 mm before correction, and about 0.058–0.068 mm after approximate correction; the lower bound is still above 0.05 mm. The conclusion does not change.
Before authorising a rebore, measure the piston skirt at the specified height and 90° to the pin with a micrometer at the same reference temperature, measure all four cylinders at the same heights and axes, verify the bore gauge against the setting ring at working temperature, and check liner protrusion, head flatness and oil clearance. Also inspect the scuffed piston and oil supply. These checks prevent reboring for a measurement artefact and confirm the correct oversize.