Sheet metal covers develop dimples at the bolt holes, which compress the gasket in one small area, and leaves long sections without enough tension. Cast aluminum won't distort that way. Chevy finally got it right with their late-model engines, with four bolts down the middle like the old Studebaker, and a captive O-ring in a groove, instead of a gasket. Unfortunately, you'd have to change cylinder heads to take advantage of the new design! I'm not even sure the late heads will fit the old block, but I don't think so!
Jerry
