Big thanks to Joe and Mike @ JMS

[2manymustangs]

I don't have a pic right now but lost the electrode from #6 spark plug. One theory is electrode got swatted and hung up on this exhaust valve. As the valve was returning to seat it decided to flower inward with the electrode as a pry bar. No pitting on piston head or valve <<< clue<<<.

My idea would be a little different, look at the other plugs and check the color/condition/age... I dont know BBC engines and which holes run hotter but it could be that either from age/detonation/bad plug it could have melted but your REAL lucky you noticed or took action when you did...

If that particular hole runs hotter in a BBC, you could be on the ragged edge with your A/F - temps...

I would dig deeper, I'm pretty sure that if the electrode broke away and blocked the valve it would have left some evidence behind on it's way out of the hole but I may be wrong, if it was one hit and gone... Hard to say but the plugs are a real good way to draw a conclusion of what led to this...

Good catch, your damn lucky THEN AGAIN, if you were looking for an excuse to build a bigger/badER mill, try a little harder...

I'm already waiting for my 429CJ to fail so I have an excuse to bump it up to 540 inches and really utilize the CJ heads if they survive...

Maybe the MILLER (Buzz/Paul/TheDr.) klan could lend some free advise here as to which hole gets hot in a BBC or what they have experienced over the decades of blowing things up / A-F / melting plugs / etc...

[Cigalert]

Some additional information....motor is 25 years old, either the riser or the exhaust manifold (or both) are bad/leaking, allowing water into the piston, compression of water knocked out the plug, upon exit/down stroke hung up valve. BTW, the electrode was found after pulling starboard exhaust. And really, no evidence of damage. To internals. And since I wasn't there for the machining rebuild process I cannot comment about any damage/pitting to the head/seat of that exhaust port.

[2manymustangs]

Some additional information....motor is 25 years old, either the riser or the exhaust manifold (or both) are bad/leaking, allowing water into the piston, compression of water knocked out the plug, upon exit/down stroke hung up valve. BTW, the electrode was found after pulling starboard exhaust. And really, no evidence of damage. To internals. And since I wasn't there for the machining rebuild process I cannot comment about any damage/pitting to the head/seat of that exhaust port.

so maybe the messed up valve was the expansion valve when you tried to compress the water...