Interesting that about the Merlins, as the BA and BSW/BSF thread systems differ in more than just diameter and pitch, having different thread angles (60 degrees and 55 degrees, from memory) and different spanner sizes.
If you work on the basis that if something can be installed wrongly, then it will be, and also that there is no underestimating human stupidity, then Merlin mechanics must have had an, er, interesting time. 1/4 BSF and 0BA aren’t that far apart.
Just to confuse matters, 0BA has a 1mm pitch and a 0BA nut fits a 6mm metric coarse thread – rather loosely and with much recuced strength. Therein lies a particularly nasty pitfall. Don’t ask me how I know this…

William

Yes, I have to agree with you on that. There is even one oddball on Rolls-Royce engines which is that the 2BA fittings have a 3/16″ BSF size hexagon which is slightly larger than the standard BA hex, great idea, because one set of spanners does the whole job? Not so, because a Merlin also has 0,1 and 3BA nuts which all have the standard BA hex. Work that one out!
Peter