Gee, you start a new thread and then do some work on your bike and look what happens to the thread!
Well, the post mortem's complete. The prognosis: The crank has fractured, clean through the crankpin. The big end bearing assy was effectively holding the two halves of the crankpin in place. With its new two piece crank, this WR250F was consequently the easiest engine ever to split the cases on! It's possible that the crack may have started at the oil feed hole.
Surprisingly, not much other damage has occured. Of course, the crank is toast, and the rod has contacted the crankwheels as they closed up though BDC. Also, the flywheel, while no doubt "gyroscoping" with the support of only one main bearing as it spun down during the emergency shutdown, lightly kissed a couple of the magneto coil cores. I'll replace the main bearings just to be safe. Otherwise, there is no damage and a minimum of metallic schrapnel to clean up.
To my admittedly untrained eye, it looks like a component defect. This is 600 mile easy single track trail-ridden WR250F that has rarely if ever clipped the rev limiter.Every other component in the engine looked like it was brand new.
Although the WRs are sold new with no warranty here, I intend to have a serious fireside chat with my local Yamaha rep about this.
I'll let you know how it goes.