All those forces ^^^^^^^^ heat, expansion, contraction, acting upon the studs and yet I've never seen a sheared stud due to those forces. Lots rusted away, cross threaded and threads stripped in the head, even a few that have fallen out, however, I have only ever seen them sheared during removal.
Simon, Your experience is obviously different to mine......7 turns or 12, makes no difference IMO, as they pretty much always shear within one or two turns of 'flush with the head'. Quality, in my opinion is more important than length..oooooer missus.
Just my own observation.
Martin