The Three Litre handbook for the later cars with external oil filter (TD21, TE and TF) advises in the handbook what oil pressure should be indicated:
It is not clear from the parts catalogue what is inside this adjusting valve and how to clear any fragments of dirt. Peter Martin kindly dismantled his spare engine to take photos. The large nut takes a 1 1/16th” socket and removes the whole assembly. It is also held in by a rubber washer so needs extracting from that once the end of the thread is reached. The flexible oil pressure pipe to the oil gauge may need to be disconnected to get at the nut. After unscrewing the adjuster, the steel ball can be released with the spring and cleaned before reassembly.
Alvis Archive 