Automotive Engine Systems and Overhauling
Engine fundamentals and overhaul
This domain tests four-stroke operation, valve timing, compression, lubrication, cooling and overhaul procedure. Know the sequence: intake, compression, power, exhaust. Valve clearance, cam timing marks, firing order, head bolt torque sequence and piston ring end gap decide whether an overhaul succeeds. Measure cylinder bore for taper and ovality, check piston-to-bore clearance, and verify ring gap in the bore with a feeler gauge. Head warpage is checked with a straight edge and feeler gauge; resurface only within the manufacturer’s limit. Replace torque-to-yield bolts when specified, use the correct coolant, and prime the oil system before first start.
Diagnosis and assembly
A compression test is a quick health check. Dry and wet readings separate ring wear from valve or head-gasket leakage. A leak-down test confirms where air escapes: intake valve, exhaust valve, rings or coolant. During assembly, oil rings and bores, fit new gaskets and seals, and rotate the engine by hand before cranking. Record clearances and torque values on the job card.
Worked example
Specified compression is 12.0 bar. A dry test gives 9.0 bar and a wet test gives 11.5 bar. The wet reading rises by 2.5 bar, so the rings and cylinder walls are the main leak path; if the wet reading stayed near 9 bar, suspect valves or the head gasket. The reasoning is that added oil temporarily seals worn rings.
Traps
Marks are lost by ignoring timing marks, fitting rings without checking end gap, reusing head bolts that are torque-to-yield, and skipping valve clearance adjustment. Never assume a low compression reading means a full rebuild before a leak-down test.