Automotive Engine Systems and Overhaul
Automotive Engine Systems and Overhaul
This domain tests four-stroke operation, engine components, cooling and lubrication, and the measurements and sequence used in an overhaul. Expect valve timing, clearances, bore wear, ring gaps, bearing clearance and torque-to-yield fasteners.
Core rules and concepts
- A four-stroke SI engine runs intake, compression, power and exhaust. A diesel draws air, compresses it and injects fuel near the end of compression for compression ignition.
- Valve timing marks must align. A one-tooth error changes compression, power and emissions.
- Measure cylinder bore at top, middle and bottom in two directions: parallel and perpendicular to the crankshaft. Taper is the top-to-bottom difference in the same plane; out-of-round is the maximum minus minimum at one height.
- Check piston ring end gap in the cylinder bore and side clearance in the groove. Measure bearing clearance with plastigauge and journal ovality with a micrometer.
- Torque cylinder heads from the centre outward in stages. Replace torque-to-yield bolts according to specification; do not reuse them.
- The cooling system needs the correct coolant mix, a working thermostat and a pressure cap. The lubrication system needs the correct oil, filter and relief valve.
Worked example
Bore readings are 80.02 and 80.03 mm at top, 80.05 and 80.06 mm at middle, and 80.04 and 80.08 mm at bottom. Maximum out-of-round is 80.08 − 80.04 = 0.04 mm at the bottom. Maximum taper is 80.08 − 80.02 = 0.06 mm. If the wear limit is 0.05 mm, the cylinder needs reboring or replacement.
Traps that cost marks
Measuring a hot engine; mixing bearing caps; failing to mark rods and caps; reusing stretch bolts; fitting a timing belt one tooth off; and checking ring gap in a worn, unhoned bore.
Key points
- Align timing marks before assembly.
- Measure bore in two planes at three heights.
- Taper and out-of-round are different measurements.
- Torque from the centre outward in stages.
- Replace torque-to-yield fasteners.