Todd is right about the springs--It has been 1969 since a head came off a valve because of valve quality--valve float for whatever reason is the culprit 99% of the time--they float because of weak springs, too heavy on the retainers, DRIVING style, too much RPM, uncontrolled burnout, etc the valve then gets smacked by the piston and that makes the head come off. I am NO fan of Indy but..they used to ( not sure about now)buy valves from BBB in Argentina and they are one of the worlds biggest makers and build the valves to the buyers specs- Indy always ordered chrome on the stems for wear protection and induction hardened tips- all plenty good enough for any race engine--hate to see your trouble but worth mentioning that Titanium retainers are the best protection and EVERY race engine should have them.