Rebore Decision from Bore Measurements and a Caliper Zero Error
The workshop is overhauling a four-cylinder petrol engine with a nominal bore of 80.00 mm. Maximum permissible wear is 0.10 mm, maximum taper 0.05 mm and maximum out-of-round 0.03 mm. Using a 75–100 mm micrometer and a telescopic gauge, the technician records cylinder 3 in the thrust and axial planes at 10 mm below the deck, mid-stroke and 10 mm above the bottom of ring travel:
axial: 80.07 / 80.04 / 80.03 thrust: 80.08 / 80.05 / 80.03
The vernier caliper used to measure the original piston reads 0.02 mm when the jaws are closed, and the piston at the skirt (90° to the pin) reads 79.94 mm. Oversize pistons are available in +0.25 mm and +0.50 mm, the specified piston-to-bore clearance is 0.03 mm, and the boring bar must leave 0.04 mm of material for honing. The other three bores are within 0.01 mm of the same figures.
1. Calculate maximum wear, taper and out-of-round for cylinder 3 and decide whether it passes. Then correct the piston measurement and work out the clearance the worn bore would actually give.
Read the worked answer
Wear: maximum reading is 80.08 mm, so wear = 80.08 − 80.00 = 0.08 mm, which is inside the 0.10 mm wear limit on its own.
Taper (top minus bottom in the same plane): axial 80.07 − 80.03 = 0.04 mm; thrust 80.08 − 80.03 = 0.05 mm, exactly at the limit - not comfortably inside it.
Out-of-round (difference between planes at the same height): top 80.08 − 80.07 = 0.01 mm; middle 80.05 − 80.04 = 0.01 mm; bottom 0.00 mm. Maximum 0.01 mm, well inside 0.03 mm.
So wear and out-of-round pass, but taper is at the limit and the bore is worn most at the top, as expected from ring loading.
Caliper correction: if the caliper reads 0.02 mm with the jaws closed, every reading is 0.02 mm too large, so the true piston diameter is 79.94 − 0.02 = 79.92 mm.
Clearance in the worn bore: 80.08 − 79.92 = 0.16 mm, against a specification of 0.03 mm. The clearance is more than five times the limit, which produces piston slap, ring breakage and oil consumption. Honing cannot fix this because it removes material and makes the clearance larger, and it cannot correct a 0.05 mm taper without removing the whole bore size. The cylinder must be bored and a new piston fitted, and because the other bores are within 0.01 mm of these figures, the whole block should be treated as due for reboring rather than one cylinder.
2. Select the oversize and calculate the finished bore and the boring-bar setting. Justify why the larger oversize is the wrong choice here.
Read the worked answer
Finished bore = piston diameter + specified clearance. With the +0.25 mm piston, nominal diameter is 80.25 mm:
finished bore = 80.25 + 0.03 = 80.28 mm.
The boring bar must leave 0.04 mm for honing, so the bar is set to:
boring size = 80.28 − 0.04 = 80.24 mm.
In practice the bore is finished to the actual piston supplied, measured at the skirt 90° to the pin, not to the nominal number on the box, because pistons are graded and manufacturing tolerance is a few thousandths of a millimetre.
Why not +0.50 mm: the maximum wear is only 0.08 mm and the taper 0.05 mm. A +0.25 mm cut removes roughly 0.12–0.13 mm of wall per side, which is enough to clean up the taper and restore a true round bore. Going to +0.50 mm removes another 0.125 mm of wall per side, and once that material is gone it cannot be replaced: the block has a maximum permissible overbore, and a +0.50 mm cut now may leave no rebuild size for the future or may take the bore beyond the manufacturer's limit, causing overheating and loss of ring seal. The +0.50 mm piston set also costs more and gives no benefit.
Before cutting, confirm the block's maximum overbore, check all four bores and record the final sizes and clearances, since the engine must end up with four matched pistons of equal mass.
3. State the measuring technique and tool-handling points that would make these readings trustworthy. Include what would go wrong if the telescopic gauge readings were transferred to a vernier caliper.
Read the worked answer
Telescopic gauge technique: clean the bore and the gauge, set the gauge slightly smaller than the bore, insert it at the correct height, then rock it through the bore's axis to find the largest dimension and lock it. Measure the locked gauge with a micrometer using the same rocking technique to find the true maximum, and read the micrometer directly - do not rely on the gauge's own scale, which is only a setting device.
Why not a vernier caliper: a caliper has line contacts and a coarse feel, typically ±0.02–0.05 mm, and it is easy to tilt it across the gauge. Transferring a 0.05 mm taper measurement to a ±0.05 mm instrument makes the result meaningless, and this particular caliper already carries a +0.02 mm zero error. Use an outside micrometer (75–100 mm range here, checked against its standard) and, for production work, a dial or digital bore gauge set from the micrometer.
Other points: measure at the specified heights with the bore truly vertical and the surface clean and dry; measure at 20 °C, or allow the block and instruments to equalise, because a cold block and a warm workshop change readings by thousandths; avoid measuring on the ring ridge or on a damaged area; record thrust and axial readings separately; check the micrometer zero with the standard before and after the job; and record piston-to-bore clearance with a feeler or dial gauge at the skirt on final assembly.