RE: OS 55AX
G'day I'm no expert on this but to me there does appear to be evidence of dry running on the inside surface of the big end bearing - darker grey streaks. Also, the crankshaft is showing quite a lot of what could be rust and there is also a stain where the rear bearing would have been seated on the crank. The stain on the crank may indicate that the bearing was actually skidding on the crank? The burned castor on the cooling fins suggests that this engine was running pretty hot too. I have never managed to get engines this caked with burned on castor. I suspect it may simply have been running too lean for too long and the cleaning cleared out its reserve of oil causing the big end to run dry and fail. As Bruce suggested, it is always a good idea to really oil up and engine after it has been cleaned.