Closer Look -
A closer look at the crank. One can see the beach marks of a progressing fatigue failure. The crack initiation appears to be in the rod bearing filet area. A high stress area due to compression/firing forces. This area is tested in a crankshaft tuning fork machine for fatigue in the lab as THE place for this kind of failure. Why it failed is a combination of weak material, or forces higher than the fatigue limit. It could be a combination of both, but even with high compression it should not have fatigued the crank. It does not appear to be torsional due to the location of the crack initiation. A metallurgical analysis of the crank would be in order to proceed to a more definitive root cause.
Overspeed events could contribute to the compressive stresses that occur at exhaust stroke on TDC, but numbers would have to be calculated based on piston and rod weight. Still more likely it was dominated by tension stresses at TDC.
One might look closely at the top of this piston to see if any preignition or detonation might have been occurring. Since fatigue is the result of accumulative overloads, 500 hrs is too many cycles for it to have accumulated loading events every rev, but only 10-20% of the time. Stuff like this is why some engines have a X minutes limit on takeoff power.
Does anyone know if all ECI cranks are certified part numbers? Are the "Y" Lycomings a certified crank, the same part number as a certified engine?
So, is there an NTSB report for this event, and would the investigator be interested in this information?
For you FAA experts out there, is there a failure reporting mechanism for this kind of thing to the FAA? or is the government best left out of these things?