Cylinder Bore at the Wear Limit on a Deadlined Top-End
A 100 cc air-cooled single is on the bench for a top-end overhaul at 48,000 km; the owner reports blue smoke and oil use. The manual gives a standard bore of 50.000–50.010 mm, a wear limit of 50.100 mm, and a new piston-to-cylinder clearance of 0.020–0.040 mm with a service limit of 0.080 mm. Using a calibrated bore gauge, you record, in the thrust axis, 50.04 mm at the top, 50.05 mm at mid-height and 50.05 mm at the bottom; in the pin axis, 50.03, 50.04 and 50.04 mm. The piston skirt, measured 90° to the pin at the recommended height, is 49.97 mm. There is no scoring, but a carbon lip is visible at the top of the bore. The workshop has a rigid hone but no boring bar; the nearest machine shop is 40 km away. The owner needs the bike tomorrow for work.
1. Calculate the piston-to-cylinder clearance, taper and out-of-round, and state whether each is within the manual limits. Show your working.
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Clearance = maximum bore − piston skirt = 50.05 − 49.97 = 0.08 mm. That is exactly the service limit of 0.080 mm, not below it. At the top of the bore the clearance is 50.04 − 49.97 = 0.07 mm. Taper in the thrust axis = 50.05 − 50.04 = 0.01 mm; out-of-round = 50.05 − 50.03 = 0.02 mm. The maximum bore of 50.05 mm is under the 50.100 mm wear limit, and the geometry figures are small, but the provided data do not include a taper or out-of-round limit, so I would check the manual before passing them. The deciding measurement is clearance: 0.08 mm is at the limit and consistent with the piston being worn. I would repeat the bore and piston measurements after calibrating the gauge and confirm the piston height and axis.
2. The owner asks you to hone the bore and fit new rings so he can ride tomorrow. Decide what you will do and justify it with the measurements and workshop constraints.
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I would not hone and fit rings as requested. A rigid hone removes glaze and can correct a very slight geometry error, but it enlarges the bore; it cannot reduce a 0.08 mm clearance. New rings in a bore with 0.08 mm clearance may fail to seat, may break, and will not cure the oil use. My decision path: first measure a new standard piston; if the clearance with the new piston is comfortably within 0.020–0.040 mm and the bore geometry is within the manual limits, I can hone for ring seating and fit the piston with correct ring end gap. If the clearance is still above the manual rebuild limit, the cylinder needs a rebore to the next oversize, which means the 40 km trip to the machine shop. The bike will not be ready tomorrow. I would explain the risk and offer a realistic date rather than a short-lived repair.
3. Describe the measuring technique you would use to make this decision trustworthy, including calibration and where the piston is measured.
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Clean the bore and piston, then calibrate the micrometer and bore gauge against a known standard at about 50.00 mm, zeroing the gauge and allowing for its anvil or reference. Measure the bore at three heights-about 10 mm below the deck, mid-stroke, and 10 mm above the bottom-in two axes at 90°: thrust and pin. Rock the bore gauge gently to find the true minimum, and record each reading. Measure the piston skirt with a micrometer 90° to the pin at the height the manual specifies, usually 10–15 mm from the skirt bottom. Keep the parts at room temperature. Check ring end gap in the bore with a feeler gauge and inspect the top lip. Repeat any reading that is inconsistent. Without calibration and correct measuring height, the 0.08 mm result is not reliable enough to decide a rebore.