Something has to be out of alignment. I have never even heard of someone breaking that shear pin. I didn't even realize it was a shear pin until I just looked it up in the diagram. One model that I had used a square lock pin, and it worked fine. Something must be binding on the shaft side, or in the driveline u-joints, in my opinion. I've engaged my pto accidentally at full rpm before, caught 1" sticks in the blower, been totally stuffed, an never popped my shear pin. Good luck on finding the actual cause.