Service and Repair Engine Mechanical Systems (Gasoline and Diesel)
What this domain tests
This domain covers engine disassembly, measurement, reassembly, timing, and torque procedures for both gasoline and diesel engines. Expect cylinder-head, valve, piston, ring, and bearing checks.
Core rules and concepts
- Clean the engine and mark components before disassembly. Keep bolts organized by location.
- Measure cylinder taper and out-of-round, piston-to-wall clearance, ring end gap, valve stem clearance, and head flatness against specifications.
- Install head bolts in the specified sequence, usually from the center outward, and use the specified lubricant and torque-plus-angle method.
- Align timing marks before removing the timing belt or chain. On diesel engines, injection timing and glow-plug operation are critical.
- Diesel compression is much higher than gasoline compression, so correct battery cranking speed and fuel quality matter.
Worked example
A new piston ring is placed in a cleaned bore and squared with the piston. The specified end gap is 0.30–0.45 mm; the measured gap is 0.55 mm. Because the gap is too large, the reasoning is that the bore may be worn or the ring may be wrong. Do not file the ring; measure the bore and rebore or replace as required. If the gap were too small, careful filing of the ring ends would be the correct direction.
Traps that cost marks
Torqueing bolts dry when the specification requires oil; reusing torque-to-yield bolts; installing bearings without checking clearance; being one tooth off on timing; and mixing valve-train parts from different cylinders.
Key points
- Measure before replacing engine parts.
- Ring end gap must be checked in the bore.
- Follow the torque sequence and lubricant.
- Align timing marks exactly.
- Diesel work requires injection and glow-plug checks.