Valve Clearance

Definition and Function

Valve clearance (or valve lash) refers to the specified gap between the tip of the valve stem and its actuating component (such as the rocker arm) when the engine is cold and the valve is fully closed. In overhead camshaft (OHC) engines where the camshaft acts directly on the valves, it refers to the clearance between the camshaft lobe and the valve tappet.

The primary purpose of setting valve clearance is to compensate for thermal expansion. During engine operation, the valves and valvetrain components (such as tappets and pushrods) expand and lengthen due to intense heat. If no clearance is set when cold, the valves will be held slightly open when hot, preventing them from seating properly against the valve seats; this results in compression loss, reduced engine power, hard starting, or even complete engine breakdown. Therefore, proper valve clearance is a critical design requirement to ensure reliable valvetrain sealing across all operating temperatures.

Specifications and Influencing Factors

Exact valve clearance values are determined through manufacturer testing and vary significantly across different vehicle models. Generally, intake valve clearance ranges from 0.20 mm to 0.40 mm, while exhaust valve clearance ranges from 0.30 mm to 0.50 mm.

Exhaust valve clearance is typically set larger than intake clearance because exhaust valves are directly exposed to scorching exhaust gases, resulting in higher temperatures and greater thermal expansion. Furthermore, diesel engines generally require larger clearances than petrol engines, and turbocharged engines require larger clearances than naturally aspirated engines. Specific clearances must always adhere to the respective vehicle's workshop manual—for instance, certain diesel engines specify 0.30–0.35 mm for intake and 0.45–0.50 mm for exhaust, whereas some petrol units require only 0.10–0.15 mm for intake and 0.15–0.20 mm for exhaust.

Hazards of Excessive and Insufficient Clearance

The precision of valve clearance directly affects overall engine performance.

When valve clearance is excessive, a metallic tapping sound occurs between valvetrain components and at the valve seat—commonly known among workshops and drivers as "tappet noise", which is particularly audible at idle. Concurrently, the valves open late and close early with reduced duration, leading to insufficient air intake, incomplete exhaust scavenging, and engine power loss. Excessive clearance also accelerates valvetrain wear and, in severe cases, may induce valve bounce or float.

When valve clearance is insufficient, the valves cannot close fully once the engine warms up, causing compression leakage, reduced power output, and, in severe cases, burnt valves and valve seats. In extreme situations, the valve may even make contact with the piston. Moreover, tight clearances hinder smooth airflow and cause higher fuel consumption.

Adjustment and Inspection

Over extended operating periods, valvetrain component wear will inevitably alter valve clearances. For example, wear between the rocker arm and the valve stem tip increases clearance, whereas "valve recession" (wear on the valve face and seat) reduces clearance. Hence, valve clearances require periodic inspection and adjustment.

The standard inspection method uses a feeler gauge. Insert a feeler gauge blade of the specified thickness between the valve stem tip and the rocker arm (or adjusting screw) and slide it back and forth—a slight, smooth drag indicates proper clearance, whereas excessive tightness or looseness indicates adjustment is required.

There are two primary adjustment mechanisms. The screw-and-locknut type adjusts clearance by turning an adjustment screw, which is straightforward to service but offers slightly lower precision; the shim type adjusts clearance by replacing shims of varying thicknesses, delivering higher precision and stability, though the procedure is more labour-intensive.

In terms of procedure, the cylinder-by-cylinder method and the two-step adjustment method are commonly used. The cylinder-by-cylinder method involves setting each cylinder's piston to top dead centre (TDC) on its compression stroke before adjusting its valves; the two-step method allows all valves to be adjusted in just two crankshaft positions, making it more widely adopted. Adjustments must always be performed on a cold engine and strictly follow the sequence and specifications outlined in the workshop manual.

Hydraulic Tappets: Maintenance-Free Technology

Modern passenger car engines have widely adopted hydraulic tappets (hydraulic lash adjusters / HLAs) . Hydraulic tappets utilise engine oil pressure to automatically self-adjust their length, compensating for thermal expansion and mechanical wear to maintain "zero-lash" operation throughout.

A hydraulic tappet consists of a plunger, check valve, and spring, altering its effective working length by regulating oil pressure within its chamber. Its primary benefit is eliminating the need for periodic manual valve clearance adjustments, while cushioning impact, reducing operating noise, and extending valvetrain service life. However, once a hydraulic tappet fails, it must be replaced rather than adjusted. As of July 2026, hydraulic tappets remain the mainstream configuration across mass-produced passenger vehicle engines.