What is the valve lash and valve clearance?

Valve lash, or valve clearance, is the small, intentional gap between the valve stem and the part that actuates it (usually a rocker arm or cam follower). It is set to a specific value when the engine is cold and can change with temperature and wear as the engine runs.

What is valve lash and valve clearance?

Valve clearance is the target specification for the gap in the valve train when the engine is at rest. Valve lash is the actual measured gap you obtain during inspection. In practice, the two terms are often used interchangeably, but the clearance is the precise value listed by the manufacturer, while lash is what you measure and verify against that spec. The gap exists to accommodate manufacturing tolerances, material expansion, and wear over time.

How temperature and design affect lash

Metal components in the valve train expand when heated. As the engine warms up, the valve stem, rocker arms, cam followers, and related parts grow slightly, which can change the gap between contact surfaces. The exact change depends on engine design. Some engines use hydraulic lifters that automatically take up minor gaps, while engines with mechanical (solid) lifters require periodic manual adjustment to maintain the specified clearance.

Solid lifters and mechanical followers

In engines with solid lifters, the lash is a fixed, adjustable gap. If the clearance is too large, you may hear tapping and lose valve efficiency; if it’s too small, the valve can’t fully close, risking poor sealing and overheating. These engines usually require cold-set lash adjustments during servicing.

Hydraulic lifters

Hydraulic lifters use oil pressure to automatically adjust the lash within a small range, reducing or eliminating the need for regular manual adjustments. Even with hydraulic lifters, some engines specify a cold lash value or a recommended routine for inspection after hard operation or major work.

Measuring and setting valve lash

To assess and adjust valve lash, technicians typically measure the gap with a feeler gauge at specific engine positions and then adjust using the appropriate mechanism (screw-type adjusters, rocker-arm nuts, or hydraulic lifter preload). The exact procedure depends on engine design and the manufacturer’s service procedure.

Most engines require adjustment with the cylinder at a precise position (often Top Dead Center on the compression stroke for the cylinder being adjusted) to ensure the valve is closed and there is no unintended pre-load from timing marks or cam lobes. Always consult the specific service manual for correct steps, tools, and torque values.

• Identify the correct cylinder and valve to adjust (for example, cylinder 1 exhaust, cylinder 1 intake, etc.).


• Rotate the engine to the specified position for valve adjustment (often TDC on the compression stroke) and inspect the valve train components.


• If using adjustable rocker arms, loosen the locknut on the adjustment screw.


• Back the screw away to create the required clearance, or preload the hydraulic lifter as specified by the manual.


• Insert the proper feeler gauge and tighten the adjustment while maintaining the indicated gauge with light drag.


• Torque any nuts or bolts to the manufacturer’s specification and recheck the lash. Rotate the engine by hand a couple of revolutions and recheck all clearances.


• Reinstall the valve cover and run the engine to operating temperature, then recheck hot clearance if the manual requires it.

In engines with hydraulic lifters, you may not be able to set a traditional “lash” value; instead, you ensure the lifters are preloaded correctly and that the system remains within the manufacturer’s acceptable range.

Concluding the adjustment process, it’s essential to verify that all valves are within spec across temperature ranges and to understand that lash can change with wear. If a valve train shows persistent noise or poor performance despite correct lash, a deeper inspection for wear, bent valves, or timing issues may be needed.

Symptoms of incorrect valve lash

ISSUES arising from improper lash can manifest as audible indicators and performance symptoms. Below are common signs to watch for, though diagnosis should always rely on manual inspection and reference to the service manual.

• Tapping or ticking valve train noise that varies with engine speed


• Rough idling, misfires, or reduced engine performance


• Loss of power, especially at higher RPM


• Difficult starting or misfiring when cold or hot


• Excessive valve train wear or unusual valve seat wear over time

If you notice these symptoms, consult a qualified technician, as incorrect lash is one of several potential causes and proper diagnosis often requires live measurement with the engine running under controlled conditions.

Practical tips and considerations

Always use the correct factory specifications for your engine and follow the prescribed procedure. Differences in engine design (two-valve vs. four-valve, overhead cam vs. pushrod, variable valve timing, and hydraulic vs. solid lifters) mean that there isn’t a universal lash value. Regular maintenance and reference to the service manual help prevent noise, valve damage, and performance loss.

Summary

Valve lash and valve clearance describe the small, intentional gap in the valve train that accommodates thermal expansion and wear. The clearance is the target spec, while lash is what you measure to verify it. Design (hydraulic vs. solid lifters) and temperature affect how this gap behaves, and proper measurement and adjustment—according to the manufacturer’s instructions—keep engines sealing properly, running smoothly, and lasting longer. If in doubt, consult the service manual or a qualified technician for engine-specific guidance.

How to calculate valve lash?

To determine the valve lash, subtract the squish from the desired valve opening lift. The cam in the graph is for a flathead, using squish of 0.002" the valve lash should be 0.0127" or 0.013". The exhaust could be set a couple thousandths larger to allow for thermal expansion.

What is a valve lash and why is it important?

Valve lash ensures that the camshaft lobe receives the most valve opening at its highest point. It compensates for thermal expansion by keeping heated metal parts separate. Valve lash is non-adjustable on some engines. Most engines have hydraulic lifters with built-in automatic lash adjustment.

How to properly set a valve lash?

Valve lash adjustment involves setting the clearance between the rocker arm and the valve stem, typically using a feeler gauge. This is done when the engine is off and after positioning the engine so that the specific cam lobe is on its "base circle," meaning the valve is closed. You adjust the rocker arm until the specified feeler gauge has a slight drag between the rocker and the valve stem.
 
Tools and preparation

• Tools: You will need a set of feeler gauges and the appropriate wrenches or sockets for your engine's adjuster nuts. 
• Safety: Disconnect the spark plug wires and remove the valve cover(s) to access the rocker arms and valve stems. 
• Engine position: Rotate the engine by hand until the desired cam lobe is on its base circle. You can do this by watching the rocker arms: 
  • To adjust the intake valve: Rotate the engine until the exhaust valve just starts to open. At this point, the intake valve is closed and the cam lobe is on its base circle. 
  • To adjust the exhaust valve: Rotate the engine until the intake valve is fully open and then slowly let it close, or rotate until the intake valve is about two-thirds closed. At this point, the exhaust valve is closed and the cam lobe is on its base circle. 

This video demonstrates the process of valve lash adjustment: 1:00Resilient Automotive PerformanceYouTube · Jan 2, 2023
Adjustment process

1. Check the gap: Place the feeler gauge with the thickness specified by the manufacturer between the rocker arm and the valve stem. 
2. Loosen the lock nut: Loosen the lock nut on the rocker arm assembly so the adjuster can move freely. 
3. Adjust the clearance: 
   • Hold the adjuster with one wrench and tighten the lock nut with another, or tighten the adjusting screw. 
   • Adjust the screw until the feeler gauge can slide between the rocker and valve stem with a slight drag, but without being forced. 
4. Tighten the lock nut: While holding the adjuster with one wrench, tighten the lock nut to secure the adjustment. 
5. Re-check: Remove the feeler gauge and double-check the clearance. It should still have a light drag. A feeler gauge that is just 1-2 thousandths of an inch thicker should not fit. 
6. Repeat: Repeat the process for each valve in the engine, making sure to position the engine correctly for each one. 

What is the valve clearance of a valve lash?

Valve Lash or Valve Clearance is the gap between the Rocker Arms and the valve tappet. This clearance must be tightly controlled. If it is too little, the valves may not seat properly.