OK I have just measured the thread that is below the taper on the steering rack end.
It's not metric
so I can't give an exact dimension, sorry.
The thread diameter is 9.6mm, and the pitch is a little coarser than 1.25mm.
The only imperial tap I have is 7/16UNF, which is bigger all around than the rod-end. Sorry I couldn't be more help. I'm sure somebody who started in engineering/mechanics earlier than me must be able to measure in Imperial.
Could you not just replace the castellated nut with a nyloc?? (Once you know what thread it is). Where were you going to get the right bolt? Could you not take the part there and get the right one?
Sean