Richard,
interesting reading indeed. Absolutely makes sense to me. They changed the bolts to a finer thread, which makes the root diameter larger, but naturally the threads themselves become shallower, which makes them prone to stripping by over-tightening. I am positively sure that this was the failure here, I can't imagine the bolts with the now larger root diameter really broke, they just stripped the threads with the now to high torque value. The threads get stripped by the axial tension which is controlled by the tightening torque and which increases with the use of lubricant, like I mentioned before. So assembling clean definitely helps here.
As there seems to be some interest in this topic, I will try and explain what is involved a bit:
In today's literature you will find tables with torque figures for different thread sizes and different kinds of lubricants. Also the surface treatment and the manufacturing process of the bolt itself and the base material makes a significant difference in friction behaviour (zinc or cad plating, blackening, cut or rolled threads, etc.). The tables are based on empiric data gained from bolting experiments.
The article shows us, that these things had indeed been considered and investigated by the BSA engineers, but I guess much of the information on specific bolt joints has been lost during the centuries.
The question of oiling or not is not a matter of better or worse, it is a matter of what pretension value had been determined for the screw joint and how this should be achieved by defining the friction characteristics and the appropriate tightening torque. If the engineers have stated to assemble a critical bolt joint dry, then you should do so. If it should be oiled, then use oil.
Nevertheless, the head bolts are not really crucial. It is first of all a matter of imposing enough surface pressure on the copper gasket to make it yield in order to creep into any recesses in the head or barrels. The plastic deformation of the head gasket is the main reason for the settling of the head bolts and the need for re-tightening. Secondly the pretension of the bolt joint has to be high enough to avoid lifting of the head when exposed to the combustion pressure. When running the engine the heat makes the copper softer and promotes the settling. The bolts loose their tension, the residual clamping force is lost and the head joint Ãs leaking.
Of course the torque value has to be small enough not to break a bolt or strip a thread, that goes without saying. The pretension and an operational tension add up when having the same direction of force, so over-tightening can also lead to a failure during operation by over-stressing the bolt!
The tightening procedure I stated is my personal experience. This way I managed to get both head joints tight without the need to disassemble and re-tighten after running the engine. I did oil the bolts slightly and it worked okay this way.