Lay a straight edge on your cover. Many have been distorted by previous over tightening. The original bolts on a GMC had Phillips head screws as well as a 7/16 hex. Tightening them with a screw driver was sufficient. If you have a leak and try to stop it by tightening the bolts more, you will only distort the cover more. If they are distorted, I straighten them with a straight edge. I refresh the bolt holes by putting a small ball peen hammer in each hole and tap it a little with another hammer. This is best accomplished by laying it on a block of wood.