never seen anything like this in my life

not completely out of the question question…

engineering guess…

I beleive those are cast aluminum manifolds. Likely c355, rr350, or something equivalent. Cast aluminum is only good to 375F-400F worst case.

my guess is you lost cooling…and cooked the exhaust. as cast aluminum temperatures rise stress creep and cracking is most common failure mode.

and the rest is history…

I'd say age and a fluke combination of several things listed.

I’m back! I’ve been down with my shoulder and arm for a little bit so I haven’t really been able to tear into this engine to figured out what happened. After removing the plugs I’ve suspected she leaned out. On this bank anyway? Cylinders 2,4,6,8 here’s some photos photos of the plugs that are 3 months old with 6-12 hours run time maybe? Nit sure. AcDelco MR43T photos are in order from cylinder 2, 4, 6,8

#2

#4

#6

#8





Stainless marine manifold top damage

Scoped all the cylinders as well looked fine and no
holes in pistons lol and checked compression cold and attempted a leak down all seems to check out perfectly fine. For what it’s worth.

I should mention this is dry exhaust with overboard dumps.

I don't think those plugs look bad at all. More likely a cooling system problem than a lean running problem.

Did you check water hoses? Maybe a clog? Water impeller starting to come apart and got stuck in that manifold?

That’s what I’m gonna tear into next I’m suspecting something along those lines