Automotive Engine Systems and Overhauling
This domain tests engine construction, the four-stroke cycle, component measurement, wear limits and overhaul assembly. Expect questions on valve timing, compression ratio, bearing clearances, piston-ring end gaps and torque-to-yield fasteners.
Core rules
An engine overhaul is a measurement-driven process: clean and inspect, measure against the manufacturer's limits, machine or replace, then assemble with specified clearances and lubricants. Valve timing must be set with the crank and cam marks aligned; one tooth off changes compression and can cause valve-to-piston contact. Main and big-end bearing clearances are checked with Plastigauge or a bore gauge and micrometer, never by feel. Torque sequences run from the centre outward in stages, and angle-torqued bolts are replaced, not reused.
Worked example
A four-cylinder bore is measured at three heights and two planes. Top reading 80.12 mm, middle 80.08 mm, bottom 80.05 mm; maximum taper is 0.07 mm. The limit is 0.05 mm, so the taper is 0.02 mm over limit. At the top, readings at 90 degrees are 80.12 mm and 80.09 mm, giving out-of-round 0.03 mm, inside the 0.04 mm limit. The correct action is a rebore with oversize pistons, not a new set of rings in the worn bore.
Traps
Mixing diameter and radius when calculating compression ratio; measuring a hot engine; reusing torque-to-yield bolts; setting ring gaps without checking the bore; and assuming a noisy tappet is always a valve-clearance fault.