do not use anything but the correct shear pin. the purpose of the shear pin is to protect the motor and any gears and/or belts in the drive to the tines. if you are shearing pins, you must be trying to till too deep on the initial pass, of there are large rocks or roots in the area that are overloading or stopping the tines. remove the cause of the shear pins shearing instead of replacing with hardended steel and it will be cheaper to buy shear pins than to rebuild or replace the tiller.

JMHO

Carl