If you have GM Aluminum Center Bolt Valve Covers, there is a material seal between the cover and the bolt. You can find them at some parts stores but... they are very expensive for what they are. Went to HD and got these same bolt hole and external size. "EVERBILT # 597-487" a pack of 4ea for $1.12 Each cover takes 4. Put them on today and they are a perfect fit.