Interesting, I don't see the "lock nuts" on the remaining rod bolts in the picture. I wonder if a rod bolt was reused once two many. The crank throw looks OK in the picture. My first thought was low oil pressure number 1. But the crank looks OK so to speak. I did see a rod like that come out of a motor that went in the water. There was more than one however. And a split cylinder. I don't think it would be RPM would it? Can that little flat head rev enough to break a rod? The carb and general flat head design, seems to make it look self governing.

Can't wait to hear what it was doing when it went!