These are high quality gaskets that works well on those hard to seal applications. The low density fiber & rubber blend gasket material seals those minor head imperfections with a low clamp force. At .3125 inch, the extra thickness can also offer slightly more clearance for roller rockers. Ideal for all those high performance applications where you take the valve covers off and on a lot, and sticking is a problem. Fits all 1965-2000 Chevy Big Blocks: 366,396,402,427,454 including GM (General Motors) Gen VI Crate Engines like the 502 and 572.