i would never use them on my personal stuff, but i did build an engine for a fellow racer using the cnc stealths. they both cracked badly on the 4 run, iirc. they had been purchased a year before deployment, as the engine sat on a stand waiting for the car to be finished. 440source warrantied them anyway, and sent him a new set. the car now runs mid nines all day long, and finished it's first season without incident. i was very impressed with how 440source handled a bad situation, which is why i continue to use them (not for heads, though). fwiw, everytime i've seen a shaft tower failure, it's been the fault of the assembler. something got overlooked. valvetrain failure in general is like that, ime. i know that hurts, but reality can be painful.