[AussieinNC]

This engine has the NON adjustable rocker arms !

The "nut" is actually the bolt that holds both the rocker arm in place and the guide plate.
The bolts MUST be tight against the cyl head or they will back out!
It does not have the stud and locking style nut as found on earlier engines.

I would also be looking very closely at the pushrod guide plate groove to make sure its not worn excessively.

Is that engine a roller cam engine?

The reason I ask is if the roller lifter set has been installed "backwards" the pushrods end up off center and usually push the rocker off of the valve tip, and or bend pushrods.