Don't know. The guy exploded 3 Stelth heads, switched to Indy EZ's and has been fine after that. Perhaps some detonation, it was/is a 14-71 blown wedge. Anyway, seems that the Stealths are not the strongest castings around. We have one set of Stealths on a Koln blown 400, and that has worked without any problems, and one with RPM's and a 8-71 (that will be switched to turbos this winter and put into a '70 Chrysler New yorker, aiming at 300 km/h top speed) and that has been fine too.