This was originally a "Chevy" rule- the intake face of a SBC is at such an angle that the valve cover bolt hole would be comprimised before any of the adjoining threads, the same is NOT true on a Hemi, the angle of the intake face is such that the threads are cut into way before the bolt hole as viewed from the valve cover. On a SBC, the valve cover bolt hole is a clear limit to how far the intake face can be cut, on a Hemi, the limit is variable, depending on how deep the threads were cut into the head, either by the factory or by others.