What Is a Cylinder Head?
The cylinder head is the upper section of the engine, bolted to the top of the engine block. It contains the combustion chambers (where fuel and air are ignited), intake and exhaust valves, valve springs and guides, the camshaft (on most modern engines), and coolant and oil passages.
On a four-cylinder engine there is one cylinder head. On V6 and V8 engines there are two — one for each bank of cylinders.
Cylinder heads on modern vehicles are almost always made from aluminium alloy, which is lighter than cast iron and conducts heat more efficiently — but is also more susceptible to warping when overheated.
Key Components Inside the Cylinder Head
- Combustion chambers: Depressions in the underside of the head where ignition occurs
- Intake and exhaust valves: Open and close to let air/fuel in and exhaust gases out
- Valve guides and seats: Precision surfaces that locate and seal the valves
- Camshaft(s): Operate the valves via lobes pressing on rocker arms or directly on buckets
- Coolant galleries: Passages that carry coolant through the head to manage temperature
- Oil galleries: Passages that deliver oil to lubricate the camshaft and valvetrain
- Spark plug threads: The head provides mounting points for the spark plugs
What Causes a Cylinder Head to Warp?
Overheating is the primary cause. When the engine overheats, the aluminium cylinder head expands unevenly — and because it can't expand freely (it's bolted to the block), it distorts. The sealing surface on the underside of the head (the mating face) becomes uneven, allowing combustion gases and coolant to escape past the head gasket.
Other causes include:
- Sudden cooling: Pouring cold water onto a hot engine can cause thermal shock
- Manufacturing defects (rare)
- Incorrect head bolt torque during a previous repair
- Combustion detonation (knock) creating excess pressure
- Coolant loss from a failing water pump, burst hose, or faulty thermostat
Signs That the Cylinder Head Is Warped or Cracked
Many symptoms overlap with a blown head gasket because the two failures often occur together:
- White smoke from the exhaust (coolant burning)
- Engine overheating
- Coolant in the oil (milky oil)
- Misfires or loss of compression in one or more cylinders
- Visible cracks in the head after removal (crack-tested by a machine shop)
- Coolant or oil seeping from the head gasket area
How Is Warping Measured?
When a cylinder head is removed, a machine shop places a precision straightedge across the mating surface and measures any gaps with a feeler gauge. Most manufacturers specify a maximum warpage of 0.05–0.10 mm. Beyond this, the head must be:
- Resurfaced (skimmed) — a machining process that removes a thin layer of metal to restore flatness. This is possible if the warp is minor.
- Replaced — if the warp is too severe, or if cracks are found, replacement is the only option.
NZ Cost Estimates
| Service | Estimated Cost (NZD) |
|---|---|
| Cylinder head removal and assessment | $300–$600 (labour) |
| Cylinder head skim/resurface (machine shop) | $150–$350 |
| Cylinder head replacement (reconditioned) | $400–$1,200 |
| Complete head gasket + head repair job | $1,500–$3,500+ |
For popular NZ vehicles like the Subaru Impreza or Toyota Corolla, reconditioned cylinder heads are usually available. Rare or performance engine heads may need to be sourced from a wrecker or remanufactured.
WoF Implications
A warped or cracked cylinder head causes symptoms (smoke, coolant loss, misfires) that can lead to a WoF failure. If the issue has led to significant oil leakage, this may also be flagged.
When to Book a Mechanic
Book a mechanic if:
- Your car has overheated — even once — and you're noticing any of the symptoms above
- You've just had a head gasket replaced and performance hasn't improved
- There are signs of coolant or oil mixing
- Compression testing shows low compression in one or more cylinders