I think it would depend entirely on the depth of thread engagement. But what's interesting to me isn't "which breaks first" but rather "why did it break at all?" And (again) to get to the bottom of this particlar case we need to know the thread conditions (oily?)....cast iron head is still going to be a fair bit cheesier (technical term....) than a grade 8 equivalent bolt? In other words, still the weakest link?.