I have seen this a bunch and it was always on very overheated engines. On a
 bunch of them the seats had came out  later  mark d

In a message dated 2/14/2012 6:42:18 A.M. Eastern Standard Time,
d23haynes57@HOTMAIL.COM writes:

The  overheating explains it. The heads started to melt and castings tend
to
explode when overheated. Try welding castings! Gotta know what you're
doing.
There is a good chance that case is also toast.  Could  be
warped-twisted-cylinder seat damaged. Replace the studs as they are  also
stretched.

Dennis