First check the flatness with a straight edge that is long enough to cover most all of the surface where the gasket sits. If over all flatness is ok. Make sure you aren't over tightening the bolts/screws that hold the cover down. If I remember right, all you need is about 5 to 7 foot pounds. As soon as you see the gasket start to squeeze out stop. It helps to put your sealant on both the cover and the head.
