Oil Consumption, a Worn Bore and a Customer Who Wants the Cheap Job
A 1.3-litre four-cylinder petrol engine from a 2014 hatchback is on the bench for inspection after 165,000 km. The owner complains of blue smoke on acceleration and oil use of about 700 mL per 1,000 km. The machinist measures each bore at three heights and in two directions, 90 degrees apart. Cylinder 2 reads 74.03 mm at the top in the thrust direction and 73.98 mm at the bottom, with out-of-round of 0.035 mm. The workshop manual gives a standard bore of 73.95–73.96 mm, maximum taper 0.015 mm, maximum out-of-round 0.010 mm and a top-ring end-gap limit of 0.70 mm. A new top ring squared in cylinder 2 with a feeler gauge shows a gap of 0.85 mm. The pistons show skirt scuffing, and the top ring groove is worn 0.08 mm wider than when new. The customer's approved budget is 35,000 rupees and covers only a 'top overhaul' - head work and new rings - not a rebore with new pistons.
1. Using the measurements, decide whether a re-ring and hone can meet the customer's request, and justify your decision with calculations.
Read the worked answer
Taper = 74.03 − 73.98 = 0.05 mm, against a limit of 0.015 mm - over three times the maximum. Out-of-round is 0.035 mm against 0.010 mm. The new ring gap is 0.85 mm against a 0.70 mm limit, and ring gap is set by bore diameter, not by the ring.
Honing removes only a few thousandths of a millimetre; it is a surface-finish operation, not a geometry correction. It cannot take out 0.05 mm of taper or 0.035 mm of ovality - that requires boring to an oversize. If you fit new rings into this bore:
- the ring must flex in and out as it travels past the taper, so it never seats and loses tension;
- the oversized gap leaks combustion pressure and oil;
- the 0.08 mm groove wear gives excessive ring side clearance and ring flutter.
The result is a repeat of the same complaint within a few thousand kilometres, plus a bore that may then need even more material removed.
Decision: decline the ring-only repair as a cure for oil consumption. Recommend a rebore to the next oversize with matching pistons and rings, or a verified low-kilometre short block. A head overhaul on its own may still be justified for the valve guides, but it will not fix the oil burning.
2. How would you explain this to the customer, and how would you protect both of you if the customer still insists on the cheaper option?
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Put the numbers on paper, not adjectives: "Your cylinder is 0.05 mm out of shape. The maker allows 0.015 mm. New rings cannot seal a bore that is already oval - they will wear to fit the oval, then leak again."
Then give two clear choices with prices and consequences:
- Full rebuild (bore to oversize, new pistons, rings, bearings as needed): higher cost, fixes the cause, carries a warranty.
- Top overhaul only (head work and rings): fits your budget, but the oil consumption will very likely return, and there is no oil-consumption warranty on it.
Explain the risk in money terms: paying twice, and possibly damaging the bore further so the later rebore costs more. Be factual - do not exaggerate to sell the big job; the measurements already make the case.
If the customer still insists, record it: a line on the job card listing the measurements against limits, the recommendation made, the customer's instruction, a clear statement that no warranty applies to oil consumption, and the customer's signature and date. That protects the customer from a surprise and protects you from a claim later. It is also fair to say honestly that you would rather not take the job at all than take it and be blamed for the outcome.
3. Before any machining is authorised, what would you recheck and what exactly would you specify to the machinist?
Read the worked answer
Recheck first. Calibrate the bore gauge against a micrometer set at the standard bore (73.95 mm), with gauge, micrometer and block at the same room temperature - a hot block and a cold gauge can account for a thousandth or two. Then re-measure every cylinder at three heights and two axes 90 degrees apart, and write the numbers down. On engines where the head bolts pull the bore out of shape, measure with a torque plate fitted.
Then specify in writing:
- bore all four cylinders to the next oversize (for example +0.25 mm) with the matching pistons in hand;
- piston-to-bore clearance taken from the manual or the piston maker's data, not guessed;
- final hone to a plateau finish suitable for the ring type, with the torque plate fitted if specified;
- ring end gaps filed to the ring maker's figures, checked with the ring squared in the bore;
- ring side clearance checked in the new ring grooves;
- each piston matched to its own bore and marked.
Finally, verify on receipt: measure each finished bore and each piston, check the gaps yourself, and keep the sheet with the job card. That sheet is your defence if a comeback happens and your proof that the machine shop did what was asked.