Shrapnel Britannica.

Well, let me begin by saying that this was about as close to the last thing that should have happened as you can get. Now, having come this far, of course we're going to fix it and get the Rising Star back on the road.
So, after the debacle that was the trip to the rally, now the damage is clear and there's good news and bad news.
The good news is that the top end is safe: not the slightest sign of seizure anywhere. The head and all the valve gear is ok, as are the cylinder barrels, pistons and piston rings. But looking a little closer I can see that the cylinder barrels have suffered a pretty severe blow and have cracked. If we need to replace them - beside the fact they are probably not easy to come by - there is the matter of added cost, including, obviously new pistons and rings.
Now for the rest of the bad news: the bottom end was almost completely starved of oil and the number 2 connecting rod (or the right hand side if you prefer) cracked and split from the big end upwards.

As it collapsed on itself, the con-rod brought the piston down, lodging it onto the top of the flywheel. This could have allegedly prevented further mayhem in the form of a totally broken con-rod poking through busted crankcases. As it is, there are two small marks where the piston's mantel hit, and I still don't know if it can be re-used.
We found bits of ground up metal everywhere, including in the SRM sump plate, so there is no alternative but to pull the engine out, split the cases and start again, basically from scratch.
The crankshaft's main journal is damaged and both journals will need to be ground down. I hate the thought of it as a principle, but I'm told it's acceptable.

We also need a new timing side bush and it will all require further machine work. Even if you think of this as a best case scenario (the cases are good, so is the the head, the gearbox, etc.), this will be a very costly fuck-up.

So what caused this anyway? The leading theory at the moment is to do with the return oil line to the oil tank. It seems we mistakenly identified the pipe shown here in the red circle:

In a typical Triumph set up, this takes a portion of the oil from the main return line, and diverts it to the top end. The rest goes back into the tank through the main return tube, which has a restricted passage precisely to allow part of the oil to flow to the head. However, on a BSA this is not the case, as the oil junction bloc at the crankcases already has a separate pipe that goes directly to the head. That being the case, the "extra" pipe shown in the red circle seemed superfluous (perhaps meant as a drip for the rear chain?) so we blanked it off with a short piece of rubber pipe and a cap. What is thought could have happened (this is still unconfirmed at this time) is that all the oil returning to the tank was too restricted and the building pressure inside the line caused some sort of lock in the oil cycle. There was always plenty of oil going to the head (hence no damage there) but for some reason the fact that not enough oil was returning to the tank caused the bottom end failure.
I'm not 100% convinced that's the whole story, but it could be a start...