Engine Mechanical Systems
The four-stroke cycle and valve train
You must be able to explain induction, compression, power and exhaust, and locate each stroke from valve position and piston direction. Both valves closed with the piston rising is compression; both slightly open around top dead centre is valve overlap. Camshaft profile, lift and duration, variable valve timing, and the difference between an interference and a freewheeling engine all matter, because a broken belt on an interference engine bends valves. Timing marks must align exactly, and belts or chains are replaced with tensioners and often water pumps as a set.
Bottom end and sealing
Understand cylinder head and block construction, gasket types, piston rings, bearing shells, crankshaft journals and thrust surfaces. Oil clearance checks use Plastigauge or micrometers; too little clearance seizes, too much knocks and drops oil pressure. Torque sequences run from the centre outwards in stages, and many modern head bolts are torque-to-yield, so they are replaced, not reused.
Lubrication and cooling
Oil pressure, flow and quality keep bearings alive; the pump, relief valve, pick-up and filter are common fault points. Cooling relies on the thermostat, radiator, water pump, fan and pressure cap, with a 50/50 coolant mix raising the boiling point under pressure.
Rule of thumb: a compression test is run with all plugs removed, throttle wide open, and an equal number of cranking strokes per cylinder; cylinders should sit within about 10 percent of each other, and a wet retest that raises compression points to rings rather than valves. Traps include confusing TDC on compression with TDC on exhaust, and reusing stretch bolts.