That may be the explanation, or it may be part of what 'hand built' means, as opposed to coming off a high volume production line. Crewe engine designer Jack Phillips says the engine looks this way because it's cast from LM8 silicon aluminum alloy, which is very difficult to machine. Mercedes or Ford blocks of this vintage are much more precisely and cleanly cast. That really shows what a cottage industry low volume car building was in England 50 years ago. As you can see, the finish of the aluminum engine block casting is surprisingly rough. The later engines (Spur and onward) look a bit more finished, and they have more complex trimmings but they are otherwise the same. A Cloud or S series V8 would look similar. The Bentley version looks exactly the same, with different logo on the valve covers. The photo below shows a rebuilt and detailed engine for a Silver Shadow.

One of the things car collectors often ask is - Does the car have matching numbers? In this essay I'll show you how to check the engine numbers on the Bentley T / Rolls-Royce Shadow series, and how they compare to the chassis numbers shown on the body, the registration, and the certificate of title.

Decoding willys stamped engine numbers