All these cars have a 17/32nd hexagon drain/filler plug, or 0.530 inch across the flats, a 14mm hex (0.5512 inch) key filed down carefully will fit. What is not so well known is that although all these plugs are 16 TPI (16 threads per inch) all Silver Cloud axles have UNS (60 deg) threads but earlier cars like Silver Dawn /Bentley R Type are Whitworth threaded profiles (55 deg). The plugs are not interchangeable and to try swapping them around will ruin your day. Exactly the same comment applies to the axle top breather plug.
Having now taken out the drain plug you are having a bad hair day trying to get it back into the axle. Simple…..the reason is that you are trying to align the plug squarely with the axle case and the thread is cross threading. Rolls-Royce always worked on the principle of making things complicated and confusing the masses. Just for you they tapped the drain plug threads some 4.5 degrees off set to the axle case line. Why? So that when the axle was in position and slightly tilted the threaded hole was truly pointing directly downwards and you are able to drain the last drip of oil.
So, to recap before attempting the technical aspect of replacing the drain plug. The plug hexagon end is laying rearwards just 4.5 degrees to the axle casing when it has engaged the threads, this gives the impression it is not going in squarely. Replace as follows, wipe the thread end to check that the thread is good, fit a new alloy sealing washer to the plug, insert your index finger into the hexagon end and screw the plug into the axle case thread. Once you have revolved it one full turn and felt it engage you should be able to screw it in as normal.
In any event the top mounted axle breather is likely to be blocked and needs removing to clean it out. The very early axle cases up to around 1951 were machined very close to the breather hexagon heads and this makes it difficult to engage a socket, beware of rounding off the hexagon corners and hold any tool squarely and firmly onto the breather. The Silver Dawn /Bentley R Type breather takes 11/16 Whit or 5/8 BSF sockets, the Silver Cloud / Bentley S type 1.125 inch AF.
The original oil seals were felt and not the type of seal that is familiar to the majority of current owners. Reasons were simple in the extreme, until the early 60’s modern rubber type oil seals perished very quickly and were not reliable, so felt did the job. The present day drawback is that the drive flanges have worn badly where the seal contacts the flange seal track and a straightforward felt seal replacement results in a torn up seal. Some oversize felt seals are available but they in turn suffer the felt seals original fate of trapping dirt and letting in water. In turn this wears the flange seal face quite quickly.
In any instance of the oil seal being replaced by a modern seal or oversize felt the drive flange will need machining, and depending upon the seal to be fitted, so will the seal housing. In practice there is little point in trying to replace an original type felt seal, even when new they would not pass an oil tightness test of inverting the axle on its flange end when the axle is full of oil. Fig 2 illustrates a comparison between a newly machined drive flange and one just taken out of service; the latter is in typical condition. Fig 3 shows the normal condition of the felt oil seal housing as removed from the axle, notice that most of the felt seal has been degraded or torn away.