The valve reliefs are in two different places. Also the C-460 head doesn't have valve cant and the piston generally has a shallower relief. The A-460 head has a cant angle and that requires that the A-460 valve relief be deeper over there on the side right above the top ring. This requires the top ring of an A-460 to be placed lower than a C-460 ring with all other factors being equal. A-460 pistons sometimes burn through at this location and even if you cut new valve reliefs, they will likely be too thin above the top ring.