Bummer.
Are the exhaust valves in your heads OEM or replacement valves? The OEM exhaust valves are two-piece and break off about where yours seems to have broken.
The pistons look like KB138's (or KB206's). If so, then they are required to have an unusually wide top ring end gap of about 0.035" in your engine application, not the more typical end gap of almost half that. Many builders miss this or think the end gap called for by KB is ridiculous, dismiss it, and gap the rings in typical fashion; the result of doing that is what you see in your engine. I am not necessarily saying this is why you had an engine failure...it's just an FYI. During disassembly, measure the top ring end gap of the other 7 top rings in their respective bores to get an idea of how end gap was set.
Of course, if the pistons are not KB hypers, then none of this info about the top ring end gap applies.
LO
P.S.: This is not a bash on hyper pistons and in fact the KB Hypers are excellent pistons when set up correctly.
