Engine Systems and Overhauling
What this domain tests
Engine principles, four-stroke operation, valve timing, clearances, overhaul sequence and measurement decisions. Questions often ask you to interpret bore or journal readings and choose the next workshop action.
Core rules and concepts
- SI engines use petrol and spark ignition; stoichiometric air-fuel ratio is about 14.7:1. CI engines use diesel and compression ignition.
- Four strokes: intake, compression, power, exhaust. Valve overlap is the period when inlet and exhaust valves are both slightly open near TDC.
- A common four-cylinder firing order is 1-3-4-2; follow the manufacturer data, not memory.
- Overhaul sequence: strip, clean, inspect, measure, decide (reuse, rebore, replace), assemble to torque and sequence, then test.
- Measure cylinder bore at top, middle and bottom in two directions 90° apart to calculate taper and ovality.
- Check ring end gap inside the bore, piston-to-bore clearance, valve guide clearance and crankshaft journal ovality.
- Torque-to-yield head bolts must be replaced; use a calibrated torque wrench and angle gauge where specified.
Worked example
Bore readings at the top are 80.04 mm and 80.08 mm at 90°. Ovality is 80.08 - 80.04 = 0.04 mm. The workshop limit is 0.02 mm, so the cylinder needs reboring or a sleeve. Reason: ovality exceeds the limit even though the absolute diameter may look close to standard.
Traps that cost marks
Measuring a dirty or worn bore; forgetting to zero the bore gauge; reusing stretch bolts; ignoring timing marks; judging wear by feel instead of instrument readings.