What would cause valve stem damage

 

Pics? What kind of damage is it?

 

The damage is on the top of the valve stem there is no other damage the rocker ARM show normal wear push rods are fine I can not get pics the heads are at the machine shop

 

The top of the rocker is not hitting the tip of the valve properly from the description you are giving. It can be caused by the fulcrum of the rocker being mounted too high or too low. It can also be caused by the rocker not being centered on the tip. Basically towards the front or rear of the truck. A pic would help. It could also be a defective foot of the rocker causing problems.