On Friday, April 3, 2015 at 9:42:40 AM UTC-4, Steve Leonard wrote:
Well, crankshafts don't like to be loaded at their output location
other than along the axis of rotation. Put a belt reduction drive
on there, and you are applying load perpendicular to that.
Interesting dynamics happen with a two cylinder in-line engine
with this setup. It is not an easy system to design.

Steve Leonard

The failure is the prop hub (receiving end of belt reduction),
not at the crankshaft. Last round was classic fatigue - nice
crystalline structure on broken part.