I got the tractor back today and saw the broken approximately 3/4" diameter shaft. That is an awfully small drive shaft, not much bigger than my 332 JD lawn tractor. It was smooth break and perfectly square to the shaft almost like it was machined although it did have some shiny triangular shaped spots on the surface in a circular pattern which was really weird. No sign of any twisting to the shaft at all, it just snapped like a piece of glass.
In regards to breaking the 3/8" chain, I think the way it was hooked to the drawbar pin must have caused more stress than normal since I also has a 5/16" grade 70 chain in line (it was looping around the stump and hooked to the 3/8" and it didn't stretch or damage it in any way. The 3/8 was just passed around the 1 1/8" diameter drawbar pin and hooked back into itself and it broke at the first link, the one the chain hook was attached to.
