Engine construction, operation and overhaul
Engine construction, operation and overhaul
This domain tests four-stroke operation, component measurement, assembly clearances and the reason for each step.
Core rules
- Four-stroke order: induction, compression, power, exhaust. Valve overlap occurs around TDC; valve clearance is set with the valve closed on the base circle.
- Measure cylinder bore at three heights and two planes; taper and out-of-round limits decide rebore. Ring end gap is measured in the bore with the ring square.
- Use plastigauge or a micrometer plus bore gauge for bearing clearance; torque main and big-end caps in sequence, usually from the centre outwards, in stages.
- Torque-to-yield head bolts are angle-torqued and replaced; do not reuse them.
- Timing marks must align before and after tensioning; turn the engine two revolutions by hand and recheck.
Worked example
For a 4-cylinder 8-valve engine with valves numbered 1–8 from the front, use the rule of nine: when valve 1 is fully open, valve 8 is closed and can be adjusted; 2 pairs with 7, 3 with 6, 4 with 5. Rotate the engine to open each valve in turn, set 0.25 mm cold with a feeler gauge that drags slightly, then recheck all clearances.
Traps
- Setting valve clearance at the wrong TDC, such as the exhaust stroke.
- Measuring ring gap with the ring at the top of a worn bore without squaring it.
- Reusing stretch bolts, or oiling head bolt threads when the specification is dry.
- Ignoring piston-to-valve clearance after skimming the head.