Some guy's (mechanics) hate what I am about to say. But here go's.
The cork gasket works well on engines without machined sealing surfaces. I always use a small amount of 3M weatherstrip adhesive (gorilla snot) on one side of the gasket to glue it to the part to install, not the engine side. I install the bolts in the holes to keep the gasket in alignment untill the glue drys. Here is the important thing, clean both surfaces completly dry of oil before you install the gasket. You see oil is viscus. If you leave oil on the surface you are trying to seal, it will never seal. NEVER!!! The oil will follow the trail just like electricity follows a wire.
Now if your engine has a smooth machined surface, dry rubber gaskets or ("or" not "and") silicone is the best sealant. But still if you leave any residue of oil on the surface it will leak.
Another tip is on valve covers, oil pans, and tappett covers. Make sure the hole you put the bolt through hasn't been damaged by the bolt. That is to say make sure the surface you wish to seal is flat. Use a small hammer ans a drift to flatten out the holes in the part you are goint to install.
Oh and don't use red silicone. All the crappy work I have ever seen had red silicone.
