Magnaflux test iz the key word here. A process called Zyglo if you can find some one doing it iz even better.( oil bath, rinse it off, and run it under ah blue light) "no flawz get past that test" BUT THE VALVEZ NEED TO BE OUT FOR EITHER TEST. I've seen cracks in the bridge between the valves, cracked valve guides, cracked rocker shaft pedestals, and as Dave sayz between the exhaust ports too. And the 906s will warp on the mating surface to the block as well. Buying 906s without having them tested first is ah crap shoot. And ah $10 boil out iz not ah test and pretty iz only good for paper weightz, Jer