Cylinder Head, Valve Train, and Block Assembly
This domain tests your grasp of the mechanical heart of the engine: the head, valves, cam, and block, and how they seal and move. Diesel heads are typically flat-deck with no combustion chamber in the head; the chamber is in the piston. Valve clearance, seat sealing, and head flatness are critical because compression is the ignition source.
Key concepts: valve lash (mechanical adjustment vs. hydraulic lifters), valve recession (seats wear and lash tightens), cam timing relative to the crank, and head bolt torque-to-yield fasteners that must be replaced and torqued in sequence. On the block side, know cylinder liner types (wet vs. dry), piston-to-wall clearance, ring end gap, and main/rod bearing clearance checked with plastigauge or micrometers.
Traps: assuming a compression loss is always rings. It can be valves, head gasket, or a cracked head. A leak-down test that hisses at the intake points to an intake valve; at the exhaust, an exhaust valve; at the crankcase, rings; bubbles in the radiator, head gasket or crack. Another trap is reusing torque-to-yield bolts or skipping the torque sequence, which causes gasket failure.
Worked example: A cylinder shows 300 psi vs. 400 psi on others. Add a few drops of oil; if compression rises, rings are suspect; if it stays the same, suspect valves or head gasket. Confirm with leak-down.
Rule of thumb: Oil raises compression → rings; oil doesn't → valves/gasket.