Apparently it's supposed to be a bolt, but I'll do some research. The PTO shaft has 2 flat surfaces with a hole and the bolt hold them together. It's has been my experience that any play (smaller diameter bolt) means quick failure.Why not start with the same shear pin and higher rpm (or lower travel speed)? Is the shear pin supposed to be a bolt or is it supposed to be a smooth shank through the shear plane? How is shear pin fit through the hole? If it's too loose and rattling around, I find they break more easily.