If you straighten the stamped steel valve cover around its edge you won't need them.
The steel around the bolt holes gets deformed from tightening, well over tightening, lol
So when you re-install it, as is, there is no holding power between the bolts, steel is already deformed down so when bolt is tight it is only tight at the bolt area not in between.
The spreaders above take the place of straightening the steel edges on the valve covers.
If you straighten the steel then spreaders are not needed
Rubber valve cover gaskets should be used if you will be pulling valve covers off and putting then back on regularly, they are reusable.
Cork seals better over time as it will swell from contact with oil inside the valve covers, but these are not reusable after about a year or so.
Working PCV system lessens chance of valve cover or oil pan gasket leaks.
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