Valve lash is the clearance between the tip of the rocker arm and the top of the valve stem when the lifter rests on the base circle of the cam lobe. It only applies to engines with mechanical lifters, since hydraulic lifters adjust themselves automatically. On a mechanical cam, though, correct valve lash is the difference between full performance and engine damage.
Why valve lash exists
Every material expands with heat. A valve in an engine under hard use grows noticeably longer than it is cold, with the stem stretching down through the guide. Without clearance, the valve would be held open permanently. The lash provides the margin that guarantees the valve really closes fully against its seat while the lobe is on the base circle.
COMP Cams explains that the lash also determines where on the lobe ramp valve motion actually begins. Set the lash tighter and the valve starts opening earlier and closing later, which effectively adds duration and shifts the timing events.
Hot versus cold lash
Cam manufacturers always quote a hot lash value, valid at normal operating temperature. Some also give a cold reference value for rough installation before the engine is first started.
The difference between hot and cold lash depends on the engine's materials and design. An aluminium block and head expand more than cast iron, so the cold lash can differ markedly from the hot figure. Always check the manufacturer's specification. Never guess.
The EO-IC method: exhaust opening, intake closing
The safest way to set the lash cylinder by cylinder is the EO-IC method (Exhaust Opening, Intake Closing). Summit Racing and JEGS describe it like this:
- Rotate the engine until the exhaust valve just begins to open on the cylinder you want to adjust. The intake lobe's base circle is now facing the lifter, so set the intake lash on that cylinder.
- Keep rotating until the intake valve just closes fully (returns to its base circle). The exhaust lobe's base circle is now facing the lifter, so set the exhaust lash on the same cylinder.
The beauty of the method is that you never risk adjusting on the ramp or the nose. You are guaranteed to be on the base circle every time, and it works on any engine configuration.
How to do it in practice
- Warm the engine to normal operating temperature (if you are setting hot lash)
- Shut the engine off and remove the valve covers
- Use a feeler gauge that matches the manufacturer's specification
- Loosen the locknut on the adjuster screw and slide the feeler gauge between the rocker arm and the valve stem
- Turn the adjuster until the gauge slides with a light drag, neither loose nor pinched
- Tighten the locknut without turning the adjuster, then recheck with the feeler gauge afterwards
A common slip is skipping the recheck after tightening the locknut. The nut can rotate the screw a touch, and then the setting is no longer true.
What happens when the lash is too tight?
If the clearance is too small, or eliminated altogether, the valve is held partly open on the base circle, and the consequences are serious. The exhaust valve sheds its heat through contact with its seat, so a valve that never closes fully cannot get rid of that heat and eventually burns. This is the most common cause of burned exhaust valves in mechanical-lifter engines. A valve that fails to seal also leaks combustion gases, which lowers compression and costs power. On top of that, tighter lash effectively means longer duration and more overlap, which can ruin idle quality and midrange performance.
What happens when the lash is too loose?
Kelford Cams puts a number on the problem: 0.1 mm of excess lash at 8,000 rpm produces a valve seat velocity of over 1,000 mm/s, which means the valve slams into its seat with enormous force. That kind of seat velocity hammers out valve seats, damages valve stems and can break springs. Excess lash also eats into the effective duration, since the valve opens later and closes earlier, pushing the power curve down the rev range. And then there is the noise. The familiar clatter of an engine with too much lash is more than an annoyance; it signals mechanical impacts that wear out the valvetrain.
How often should you check?
With modern components such as hardened valves, screw-in rocker studs and chrome moly pushrods, the lash does not wander on its own. It should still be checked:
- After the first run with a new cam (once it is broken in and cooled down)
- At the start of every season if the engine sees track use
- Immediately if the engine starts sounding different, since a change in lash can be an early sign of cam wear or a stretching valve
Summary
Valve lash is no old-fashioned maintenance chore. It is a precision setting that directly affects the engine's duration, timing and service life. The correct lash, checked with the right method at the right temperature, costs nothing but a little time. The wrong lash costs valves, camshafts and power.
Contact Meksta. We always state the correct valve lash for our camshafts.