To adjust a valve lash on a Chevy, start when the engine is fully cooled. Mark the balancer every 90 degrees with timing tape and coat the surface with oil or lube before beginning. Take off the valve covers and remove the spark plugs. Move the first cylinder to the top dead center compression stroke. To move the cylinder to the top dead center compression stroke, pay attention to the opening and closing of the exhaust and intake.
When the intake is closed, the top dead center is reached. Set the intake and exhaust valves to zero lash by simultaneously spinning the pushrod and tightening the rocker. The pushrod should lack lateral movement after tightening. This indicates zero lash. Use another half turn to tighten the rocker and preload the rocker arm, pushrod and lifter.
The intake valve is open on the second cylinder. Rotate the engine another 90 degrees; this shows that the engine is set on the camshaft lobe's base circle. Repeat the pushrod and rocker adjustments for that cylinder, then move on and repeat it for the remaining cylinders.
The valve lash happens when the rocker arm clears the valve stem. Too much clearance can harm the valvetrain or produce engine noise. Not enough clearance results in poor performance of the vehicle.