Engine Systems and Overhaul
What this domain tests
The Engine Systems and Overhaul domain covers the four-stroke cycle (induction, compression, power, exhaust), valve timing, firing order, measurement of wear, and correct reassembly to specification. Inline four-cylinder engines normally fire 1-3-4-2; pistons 1 and 4 rise together, but only one is on compression. Valve overlap and camshaft timing control breathing, and a jumped timing belt shows up as low, uneven compression and rough running.
Measurement and assembly rules
- Ring end gap is checked with the ring square in the bore at the specified depth, using a feeler gauge; file the ends only if it is below specification.
- Bearing clearance uses Plastigauge across the journal, torqued to spec, then compared with the scale, typically 0.025-0.065 mm.
- Cylinder head bolts are tightened from the centre outwards in two or three stages; torque-to-yield bolts must be replaced, not reused.
- Ring gaps are staggered roughly 120 degrees apart, bores are honed to about a 45-degree cross-hatch, and everything is washed clean before assembly.
- Pre-lube the oil system before the first start.
Worked example
A four-cylinder engine has a bore of 75 mm and a stroke of 84 mm. Swept area = 0.7854 × 7.5² = 44.2 cm². Volume per cylinder = 44.2 × 8.4 = 371 cm³. Total displacement = 371 × 4 = 1485 cm³, so the engine is a 1.5-litre unit. Millimetres must be converted to centimetres before applying the formula, or the answer will be out by a factor of a thousand.
Traps that cost marks
- Measuring ring gap at the worn top of the bore instead of the specified depth.
- Lining up all ring gaps on one side.
- Reusing stretch head bolts or skipping the final angle stage.
- Cranking a freshly built engine without oil pressure.