Deciding Between Honing and Reboring After Overheat
A four-cylinder petrol engine (bore 73.0 mm, stroke 82.0 mm, stated compression ratio 9.2:1) is stripped after overheating. The cylinder bores measure 73.12, 73.15, 73.14 and 73.13 mm. Taper is 0.03 mm and out-of-round is 0.02 mm; the manufacturer's limit is 0.02 mm taper and 0.015 mm out-of-round. Oversize pistons are available in +0.25 mm and +0.50 mm. The cylinder head is warped by 0.12 mm (maximum 0.05 mm) and the valve seats need cutting. The customer wants to 'just hone the bores and fit new rings' to save money, and asks whether the head can be used without resurfacing because the engine 'only overheated once'. The workshop has a cylinder bore gauge, micrometer, straight edge and feeler gauges, but no flow bench. The vehicle is a family car and the owner plans to keep it for at least five years.
1. Calculate the standard swept volume of one cylinder and the total engine displacement, and decide whether the bores can be honed to standard. Show your working.
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Swept volume per cylinder: V = π/4 × b² × s = 0.7854 × 73.0² × 82.0 = 0.7854 × 5329 × 82 = 343,203 mm³ = 343.2 cm³. Total displacement = 343.2 × 4 = 1372.8 cm³, about 1.37 L. The largest bore is 73.15 mm, which is 0.15 mm over standard. Taper is 0.03 mm and out-of-round is 0.02 mm, both above the limits of 0.02 mm and 0.015 mm. Honing to standard size cannot correct 0.15 mm of wear; it would only clean the surface. A rebore to +0.25 mm (final bore 73.25 mm) removes 0.10 mm from the largest bore diameter (0.05 mm per side), enough to clean up the taper and out-of-round, provided the specified piston clearance is maintained. If +0.25 does not clean up all bores, use +0.50 mm. The decision should be based on the largest bore and the specified clearance, not on the average.
2. Explain why hone-and-rings is not acceptable, using the taper and out-of-round figures, and what could happen if it is done.
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Hone-and-rings only restores surface finish. The rings must seal against a round, straight cylinder. With 0.03 mm taper and 0.02 mm out-of-round, ring tension varies around the bore; the ring cannot follow the shape, so compression leaks, oil is not wiped from the walls, and blow-by increases. The new rings may never bed in, and the engine can smoke, consume oil, and lose power. In severe cases the piston can scuff. The correct repair is to bore and hone to the next oversize with the piston clearance specified by the manufacturer, using a torque plate if required, and fit matching pistons and rings. The customer's five-year ownership plan makes a proper repair cheaper than a repeat strip.
3. The head is warped 0.12 mm with a 0.05 mm limit. Calculate the minimum material removal to restore flatness and explain two consequences of resurfacing, plus why 'only overheated once' is irrelevant.
Read the worked answer
Minimum removal: at least 0.12 mm to remove the warp, plus a cleanup allowance, so a cut of about 0.12-0.15 mm is needed, but never exceed the manufacturer's maximum skim. Consequences: (1) combustion chamber volume decreases, so compression ratio rises; the engine may detonate or require higher-octane fuel, and valve-to-piston clearance may reduce; (2) cam timing and valve train geometry can change if the head height changes significantly. I would cc the chambers after machining and calculate the new CR, check piston-to-valve clearance, and pressure-test/crack-test the head. 'Only overheated once' is irrelevant: a single severe overheat can warp an aluminium head by 0.12 mm, which is more than twice the 0.05 mm limit. Reusing it without resurfacing risks gasket failure and coolant leaks.