Originally Posted by Diesel911

I also have a 300D but on mine there are studs going into the Cylinder Head and Nuts that hold down the Valve Cover.

If the Nut is turning around the stud and the Stud is not moving there are several Threads/posts on how do deal with them.

If it is only the nut that is turning in general you wedge something under the nut and pry up while you turn the nut off in hopes of catching some good threads and getting the nut off.

Grabbing the nut with a vice grips and pulling up on it and trying to back it out.

If you have a Dremel tool and small grinding bits you could grind the nut in two.

If there is enought room for it to fit there is also a tool; a Nut Splitter that will crack the nut on one side.

After that you replace what ever is damaged.