Bore Readings Taken With a Dropped Micrometer
A 1.2-litre four-cylinder petrol engine is stripped for overhaul after overheating. The workshop's 50–75 mm outside micrometer was dropped last month; checked against a certified 50.00 mm gauge block it now reads 50.03 mm, and the error is treated as constant across its range. The dial bore gauge was zeroed at the nominal 71.00 mm using that micrometer. Bore readings recorded at three heights on two axes are 71.06 and 71.02 at 10 mm below the deck, 71.04 and 71.01 at mid-stroke, and 71.03 and 71.01 above the ring travel. Service data gives maximum taper 0.04 mm, maximum out-of-round 0.02 mm, standard piston-to-bore clearance 0.02–0.04 mm, and 0.25 mm oversize pistons and rings are available from the dealer. The head uses torque-to-yield bolts tightened to 40 N·m plus two 90 degree stages. The technician plans to reuse the bolts because the engine has run only 62,000 km, and to pull the angle stages by eye with the same uncalibrated torque wrench.
1. State what the micrometer error does to the recorded diameters, recalculate taper and out-of-round, and say which recorded figures you can and cannot trust.
Read the worked answer
The micrometer reads 0.03 mm high, so when the bore gauge was zeroed, the gap it actually saw was 70.97 mm, not 71.00 mm. Every bore reading is therefore 0.03 mm too large; subtract 0.03 from all six. Corrected set: 71.03/70.99, 71.01/70.98, 71.00/70.98.
Taper is the largest minus the smallest diameter along one axis: recorded 71.06 minus 71.01 equals 0.05; corrected 71.03 minus 70.98 equals 0.05. Out-of-round is the difference between the two axes at the same height: 71.06 minus 71.02 is 0.04, and corrected 71.03 minus 70.99 is also 0.04. A constant zero error shifts only the absolute size; any difference between two readings is unaffected. So the taper and ovality conclusions stand.
What cannot be trusted is the absolute diameter, and that is exactly what the piston-clearance decision needs: the whole clearance band is 0.02 mm wide while the gauge error is 0.03 mm, larger than the tolerance itself. Clean the anvils, recheck against the gauge block or replace the micrometer, re-zero the bore gauge and repeat the absolute measurements before ordering a piston.
2. Recommend hone or rebore, with the numbers behind your decision, and state exactly how you would size and finish the bore.
Read the worked answer
Both limits are exceeded on the corrected figures: taper 0.05 against 0.04, out-of-round 0.04 against 0.02. Honing removes only hundredths of a millimetre and cannot take out 0.04 mm of ovality without also pushing the bore past the ceiling for a standard piston - a 70.97 mm standard piston plus 0.04 mm maximum clearance gives 71.01 mm, and the corrected bore is already 71.03 mm. So rebore to 0.25 mm oversize.
Do not bore to a nominal 71.25 mm. Measure each new piston across the skirt about 10 mm up from the bottom, at right angles to the pin, add the specified clearance, and finish each hole to its own piston. Bore with a torque plate fitted and the main caps torqued if the data requires it, leaving 0.03–0.04 mm of stock for the final hone, and hone to a cross-hatch near 45 degrees with the stone grade matched to the ring type - chrome or moly rings need different finishes.
After honing, wash the block with hot soapy water and oil immediately, then re-measure all six readings per cylinder for taper and out-of-round and record them on the job card.
3. Comment on the plan to reuse the head bolts and to pull the final 90 degree stages by eye.
Read the worked answer
Reject both. Torque-to-yield bolts are tightened past their elastic limit; once released they do not return to length, so reusing them loses clamp load on a hot aluminium head, which is how head gaskets fail. The specification generally says replace, or gives a maximum free length to measure with a gauge; 62,000 km is irrelevant if the bolts have already been stretched once. Fit new bolts, lubricate the threads and under-head as specified, and follow the tightening sequence.
Use the torque wrench only for the 40 N·m stage. The two 90 degree stages must be turned with an angle gauge or torque-angle wrench. Pulling by eye typically loses 10–20 degrees, which is a large fraction of the clamp load the designer intended, and it is not repeatable from bolt to bolt.
The dropped torque wrench is not itself proof of error, but it was also used to check the micrometer, so have it calibrated or verified before you rely on the 40 N·m figure. Record bolt batch, torque values and angles.