Oil Consumption on a 38,000 km Underbone
A 2018 125 cc underbone with 38,000 km comes in using about 500 mL of engine oil every 600 km. The owner reports blue smoke after long downhills and at startup, and the exhaust smells oily. A dry compression test gives 1.05 MPa (spec 1.15 to 1.45 MPa); a wet test with 5 mL of oil added gives 1.32 MPa. Leak-down at top dead centre shows 30 percent loss with air clearly heard at the oil filler and none at the throttle body or exhaust. On teardown the bore measures 52.41 mm at the top, 52.32 mm at the bottom and 52.36 mm across, with visible vertical scoring. Piston-to-bore clearance with the old piston is 0.11 mm. Top ring end gap is 0.55 mm against a 0.35 mm limit. Valve stem-to-guide clearance is within spec on both valves, but the stem seals are hard and flattened. The customer wants only new rings fitted because it is cheaper, and the shop is quiet this week.
1. Interpret the test results. What do they tell you, and what other test would you run?
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Dry compression at 1.05 MPa is below the 1.15 to 1.45 MPa spec, and the wet test rising to 1.32 MPa shows the added oil temporarily sealed the rings, so most of the loss is past the piston rather than the valves. The 30 percent leak-down with air at the oil filler confirms it: air is escaping into the crankcase. The bore numbers agree. Taper is 0.09 mm top to bottom against a typical 0.05 mm limit, with scoring and a ring end gap of 0.55 mm against 0.35 mm. Rings cannot seal a tapered, scored wall, so oil is left behind on the cylinder and burns, which explains 500 mL per 600 km. The smoke at startup and on long downhills is a second, separate fault: hardened, flattened stem seals let oil down the guides under high vacuum. The guides themselves are in spec, so seals alone are needed there. Still worth running: a cylinder leak-down repeated after a few drops of oil, to separate ring from valve loss on the record.
2. Rings only or rebore? Justify with the measurements and list the head work you would do.
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Rings only would fail. The bore is tapered 0.09 mm against a limit near 0.05 mm, is out of round and scored. New rings would conform briefly and then lose tension. A 0.55 mm end gap confirms the rings are finished, and 0.11 mm piston clearance is past the service limit, so the piston is worn too. Recommended: bore or hone to the next oversize (+0.25 mm), fit a matched oversize piston and ring set, and set piston-to-bore clearance and ring end gaps to the manual figures, checking gap in the bore before assembly. Hone to roughly a 45-degree crosshatch, then scrub with soap and hot water until a white cloth stays clean, and oil the bore immediately. Head work: replace the hardened stem seals, re-verify stem-to-guide clearance, cut or lap the seats, check valve face margin and spring free length, and inspect the cam, rocker pads and cam chain while it is apart.
3. The customer insists on rings only. What do you say, and what do you write?
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Show him the numbers on the bench: 0.09 mm taper against a 0.05 mm limit, 0.55 mm ring gap against 0.35 mm, and the scored bore. Explain the outcome in practical terms: rings only will likely burn oil again within a few thousand kilometres because new rings cannot seal against a tapered wall, and the shop cannot warrant that repair. Do not refuse the job. Give two written options: rings only, explicitly without any oil-consumption warranty, and the rebore with a stated workmanship warranty, for example 6 months or 10,000 km. Let him choose, and have him sign and date the chosen line and initial the declined recommendation. On the job order record the measurements, both quotations, the explanation given, and any price change approved before extra work. That signature is what protects both sides if he returns in three months with the same complaint.