What Are Engine Valves?
Engine valves are precision-machined components that open and close to control the flow of gases into and out of the engine's combustion chambers. There are two types:
- Intake valves: Open to allow the air/fuel mixture (or just air on direct-injection engines) into the cylinder during the intake stroke
- Exhaust valves: Open to release burnt combustion gases from the cylinder during the exhaust stroke
Each cylinder has at least two valves — one intake, one exhaust. Many modern engines use four valves per cylinder (two intake, two exhaust), which improves airflow and combustion efficiency. A four-cylinder engine with four valves per cylinder has 16 valves in total.
How Do Valves Work?
Valves are located in the cylinder head and are operated by the camshaft. Each valve consists of:
- A stem (the long thin shaft that slides through the valve guide)
- A head (the mushroom-shaped end that forms the seal against the valve seat)
- A valve spring that holds the valve closed
- A collet/keeper that locks the spring to the stem
When a camshaft lobe rotates against a rocker arm or directly onto a bucket/follower, it pushes the valve open against spring pressure. When the lobe passes, the spring closes the valve again.
Valves operate in temperatures exceeding 400°C (exhaust valves especially) and must maintain a perfect seal against their seats under high combustion pressures.
What Is Valve Clearance?
Valve clearance (also called tappet clearance or valve lash) is the small gap between the valve stem and the camshaft lobe or rocker arm. This gap exists because metal expands when hot — a clearance set when cold allows for expansion at operating temperature.
If clearance is too large: The valve opens late and closes early, reducing engine efficiency and creating a tapping noise.
If clearance is too small (or zero): The valve can't fully close. This allows combustion heat to escape through the small gap, causing the valve head to overheat and eventually burn — a condition called a burnt valve. Burnt valves require cylinder head removal and valve replacement.
Does Your Car Need Valve Clearance Adjustment?
It depends on the engine design:
- Hydraulic valve lifters/tappets: Self-adjusting using oil pressure. Most modern engines use these — no periodic adjustment needed.
- Mechanical (solid) tappets: Require periodic adjustment. Common on Toyota engines (especially older Corollas and Hilux), Honda older SOHC engines, and many Mazda engines.
Check your owner's manual or ask a mechanic. Adjustment intervals for mechanical tappet engines are typically 40,000–80,000 km or when a tapping noise develops.
What Does a Valve Clearance Service Involve?
A valve clearance adjustment (often called a "tappet adjustment"):
- Engine is allowed to cool (adjustment is done cold on most engines)
- Valve cover is removed
- Each valve clearance is measured with a feeler gauge
- Out-of-specification clearances are adjusted by replacing shims (disc-type adjusters) or adjusting locking screws (screw-type adjusters) on the rocker arms
- Valve cover is refitted with a new gasket
On shim-type engines (common on many Japanese engines), this is a painstaking process as shims must be measured and the correct thickness sourced. Labour time: typically 2–4 hours.
Burnt Valves
A burnt valve — caused by a valve that couldn't close properly (insufficient clearance, worn valve seat, or broken spring) — causes:
- Misfire in the affected cylinder
- Low compression in that cylinder
- Rough idle and loss of power
- Blue or grey smoke from the exhaust
Repair requires removing the cylinder head and either lapping the valve (minor case) or replacing the valve and/or valve seat. This is major work and often combined with a head gasket replacement.
NZ Cost Estimates
| Service | Estimated Cost (NZD) |
|---|---|
| Valve clearance adjustment (4-cyl, screw-type) | $150–$350 |
| Valve clearance adjustment (4-cyl, shim-type) | $300–$600 |
| Valve clearance adjustment (6-cyl) | $400–$800 |
| Burnt valve repair (head removal) | $800–$2,000 |
WoF Implications
Loose valve clearances causing misfires or burnt valves causing excessive smoke can result in WoF-related issues. NZTA requires that vehicles don't produce excessive exhaust emissions.
When to Book a Mechanic
Book a mechanic if:
- You hear a persistent tapping or ticking from the top of the engine
- Your engine is misfiring or running rough
- You're due for a major service and have a mechanical-tappet engine
- Compression testing has revealed low compression in a cylinder