Stuart

Occasionally these engines new have insufficient lube in the crankcase and, as the bottom end is mostly "waste oil" lubed, the rod bushing welds to the crank and spins in the rod before sufficient lube arrives. They will run without load in this condition for a while, but will fail as yours did when loaded.

The prevention is to add oil directly to the crankcase (via the carb) to ensure everything is lubed before the first run.