From years of building RST engines, i find that the std bolts are prone to snapping due to there thinner shank. This is exagerated even more if the water leaks into the bolt area causing premature corrosion.

I understand that there are two different materials here, cast iron and aluminium, but the cossie is the same setup and that doesnt use stretchbolts, it uses bolts with a greater tensile strength that are less prone to stretch.