Cylinder Head, Valve Train, and Engine Block Diagnosis and Repair
What the candidate must understand
This domain covers the hard parts: head castings, valves, seats, guides, springs, pushrods, rockers, camshaft, lifters, liners, pistons, rings, bearings, and the block itself. You should understand valve adjustment procedures (overhead valve lash vs. overhead cam), the role of valve stem seals, and how a worn guide or burned seat shows up as low compression, oil consumption, or a misfire. Know that a leak-down test distinguishes where compression escapes: intake valve, exhaust valve, head gasket, or rings. Air heard at the intake or exhaust identifies a valve; bubbles in the radiator or air at an adjacent cylinder identifies the head gasket; crankcase air identifies rings or a scored liner.
Understand wet vs. dry compression tests: a wet test that raises compression points to rings; one that doesn't points to valves or head gasket. Know the difference between a cracked head and a warped head, and why flatness and pressure testing matter after an overheat.
Traps
Questions often confuse valve lash too tight (holds valves open, causes rough running and burned seats) with too loose (noise and reduced lift). Another classic trap is blaming rings when the leak-down noise comes from the intake or exhaust. Watch for answers that recommend machining a warped head without checking for cracks, or that ignore torque-to-yield fasteners and reuse them.
Worked example / rule of thumb
Rule of thumb: listen where the air escapes. A cylinder with 30% leak-down and audible hiss at the oil fill means rings or liner; the same reading with a hiss at the exhaust manifold means an exhaust valve. Match the noise to the repair before disassembly.
Key points
- Leak-down noise location identifies the leaking path.
- Wet compression rise indicates ring sealing; no rise points to valves or gasket.
- Tight lash can hold valves open and burn seats; loose lash causes noise.
- Pressure-test and check flatness after any overheat.
- Follow torque-to-yield and fastener replacement rules.