So with theblock split it was easy to very quickly diagnose that the balancer rod bearing had completed collapsed, I could wiggle the balancer rod in just about every direction by a good 2/3mm!
Not the best photo but from peering down into the cylinder bore, it all looks ok, no scores in the bores etc.
Balancing rod removed. Note the heat marks on the top of the arch.
Photo showing the 'fractured' split on con rod
Note the bearing shells had worked their way around 90 degrees to the split of the conrod. Also note the heat marks on both sides.
Bottom of con rod out, same heat marks on top of arch.
Look at the machined surface on the crankshaft where the balancing rod runs, it appears that the surface seems to have been worn away. New crank?