Engine Systems: Construction, Operation and Service
This domain covers petrol and diesel engine construction, the four-stroke cycle, valve train, cooling and lubrication, and routine service adjustments. Expect questions on valve clearance, timing marks, head-bolt torque and the difference between TDC compression and TDC exhaust.
Core rules
- Four strokes: intake, compression, power, exhaust. The camshaft turns at half crankshaft speed in a four-stroke engine.
- Valve clearance is measured cold with a feeler gauge between the rocker and valve stem, or between cam and bucket, as specified. Adjust by screw and locknut or by changing shims.
- Set cylinder 1 at TDC on the compression stroke before checking valve clearance or timing. Confirm by both valves being closed and the timing marks aligned.
- Head bolts often use a torque-to-yield sequence: torque to a low value, then angle-tighten. Replace stretch bolts where specified.
- Timing belt or chain marks must align with the engine at TDC. Rotate the engine twice by hand and recheck before starting.
- Cooling system: correct coolant mix, pressure cap and thermostat. Lubrication: correct oil grade, filter change and oil pressure warning response.
Worked example
The intake valve clearance specification is 0.20 mm. A 0.20 mm feeler slides with slight drag, a 0.25 mm feeler enters and a 0.30 mm feeler does not. The clearance is about 0.25 mm, which is within a 0.15–0.25 mm range but at the limit; record it and recheck at the next service.
Traps
- Setting timing at TDC exhaust instead of TDC compression.
- Overtightening head bolts or reusing torque-to-yield bolts.
- Adjusting valve clearance with the engine hot when the specification is cold.
- Ignoring a worn timing belt and bending valves.