the heads do crack . if you know why tell me? i have seen lot of them that are junk. and boat engine run cooler than car the themo shock is from cool lake water and the chamber heat and the fact that the heads is heat induction on the ex side to make it harder all this makes them crack . rigth at the seats .but have used them same heads on the street and not had that happpen.just know to look out for this i did work on boats and seen this