What is Valve Lashing
 

I just heard about this and am wondering what it is, what kind of symptoms come with it, and how do you correct it? Thanks

Valve lash is the space between the valve and the rocker arm. When this get too far out of spec. you start having a ticking sound coming from the valve train. Depending on your truck, the general specs for valve lash is 0.010 for intake valves and 0.020 for exhaust valves.

In order to set valve lash, find TDC for cylinder 1, and measure the clearance for Intake 1, 2, and 4, and Exhaust 1, 3, and 5. If the lash is greater than spec, loosen the nut and turn the adjusting screw until clearance is correct, the retighten the lock nut. Then, turn the engine over 360*, measure the clearance for Intake 3, 5, and 6, and Exhaust 2, 4, and 6. If lash is greater than spec, then loosen the lock nut and tighten the adjuster to spec, the retighten the lock nut.

Make sure to be careful when removing the valve cover, as some trucks have wiring integrated in the valve cover gasket. You may also want a second valve cover gasket, just in case.

Others will chime in.

Have an 01 2500 ext cab 4x4

All (to my knowledge) non-hydraulic lifter engines have valve lash. It is a needed gap between the push rod and rocker arm to allow for thermal expansion. The push rod lengthens as the engine and its parts get hot. If there was zero lash when cold, the valves would stay slightly open when the engine gets to operating temperatures.

Heres how you can verify that your engine is TDC and 360* out. This is the Fuel Pump Gear you have to remove breather tube assembly first. Its on the driverside front cover.
Attachment 41963

the push rod does lengthen some, but most of the expansion comes from the valves growing. that's why the exhaust valve has more clearance, because it grows more than the intake valve (hotter).