Could to much torque be the reasoning behind so many hairline K-model head fractures ?
I've had some thoughts about these cracks over the years, I've got about 7 heads, only 2 are not cracked... Upon inspection, the 2 uncracked ones have had the KR radiused pocket relief done in the past...
The std casting has quite a brutish and squarish transition between the roof and the vertical side on the outside of the valves, this surely must promotes cracking along the transition. But not explain why the cracks appear in the 1st place... It's also quite a complicated casting to make, having a core unlike most other sv heads I know of, so I'm quite sure they experienced a lot of casting rejects as well as ending up machining some which were not as perfect as they seemed.
I came to the conclusion it's the double decker fin that promotes cracking. It's further away from the primary heat, and when the engine is stopped, it's the 1st one to cool and contract, hence internal casting stresses that will result in std heads cracks at their weakest point.
The raduised KR type has a smother transition, stesses are still there but the crack having no starting point does not appear. Late 60's racers often got rid of the double decker fin as well by cutting it on a miller...