ok stroker guru's, gig this one

The modern blocks are cast much different. The aftermarket supports them with special sleves and deck plates to make them stringer, or to go with bigger than factory bores. I sleeve blocks when the sonic test says one or two holes are thin even if there's no particular problem visible. The sleeve material is better and you end with a stronger more consistent bore. This is why NHRA mandates a max number of sleeves. Otherwise racers would be doing just that...