There are three different sizes of screw-in core plugs on 6.25L and 6.75L Rolls-Royce V8 engines.  These core plugs can be very tight.  It is important that these sockets be used with an impact gun held perpendicularly and very firmly or else fouling the plug holes will result.  It will be very difficult to hold these straight using a ratchet or a wrench.  These sockets are made from solid steel with pressed-in pins made from hardened drill rod.