I believe D.C is referring to countersinking the thru-the-case bolt holes to provide a place for the Viton (green) O-rings at both ends of the bolts? You
chamfer the bolt hole as a grove for the o-ring to sit. Otherwise, the o-ring interferes with the washer which should lay flat against the case or is just gets squeezed out of place and doesn't seal. Yes, it's a
known issue. Getting this wrong results in some pretty hefty leaks.