I can't get any standard ones so this is my best option so far. The 304 stainless is quite hard and not overly brittle. I might just try to torque up some nuts twice the specified tension on the booker rod and see if I can snap it. If it doesn't work I'll have to go shopping again.

Cheers.

P.S. Shear strength of 8mm booker rod is 500 MPA. Any idea what 8.8 or 10.9 is?