can a new rocker cover affect the performance

[b.fitzs]

,i'm planning to fit a new one

[The Marcos Graveyard]

No.

It's just there to stop oil escaping from under the rocker cover, you can stick the cork gasket to the cover with gasket cement, makes it easier to put on the rocker cover and is reusable.

[Mini]

Tip.

Stick the gasket to the head rather than the cover, it stops oil dribbling over the head when you take the cover off.

Cheers, :D