Nope valve seat is there.

huge, That Comp Cam 270H is what caused the valve failure, It is a street/race car cam that was not designed to run mid to high rpm's for an extended period of time such as found in a marine engine, It has a very steep ramp to it that does not let the valve sit on the seat long enough to transfer heat.

There you go! Listen to Lee! --- Jer

Your pictures bring back bad memories. Grenaded my 540 after about 50 hours, looked almost identical to your except it was #8 cylinder. It turned out to be water reversion in my case. Plus I was preheating water in my Powerflow plus manifolds which Helped to cool the reverted water even more. Brass valve guides started to go brittle, pieces started to get sucked into engine. Then at 6000 r.p.m. I spit most of an exhaust valve out the manifold the balance stuck in the head. I guess I was pretty lucky had no damage to the block, Canfield aluminum head had to be reworked extensivly, 3 new pistons all new rings, bearings, ect. I was using Manley severe duty valves at the time. I hope all goes well in the rebuild.

the width of the contact area between the seat and the valve has to be wide enough to transfer heat away from the valve as well since it is the only time the exhaust valve is cooled.
fran

A wide seat is the ticket for a supercharged engine, But on a N/A engine it will hurt the flow and is not required, The Ex valves in a N/A engine will not get nearly as hot as the supercharged engine will, Unless you have a cam grind like the Comp Cams Magnum series 268H 270H 286H 292H 305H

Things to check beside what was stated above is
valve clearance
Valve Keepers & locks,
Valve spring bind

What was the compression ratio.

Looking at piston it looks like it has melted also. My favorite topic is detonation & when the motor goes in detonation all kind of things can break. I have been lucky when I have detonated an engine and broken parts off the pistons. When you have loose parts flying around anythng else can happen.

My vote is on detonation. Are any of the other exhaust valve tuliped?

Lee,
you said the exhaust valve failed from heat, you agree that the wider seat transfers more heat to cool the valve, and then you said it's not required in a natural breathing engine ? i'm not trying to change your opinion, but to me, a wider seat for less prone exhaust valve failure is a good trade off for a little flow, a few horses, and the price of a new engine.

Excaleagle42, After disassembling 4 different blown up big blocks all with Inconel exhaust valve failure and all had Comp cam 270H-292H cams in them I thought it was kind of strange so I made a call to Comp Cams and talked to a camshaft engineer who told me these cams will not work in a marine engine and will cause valve failure and after examining a cam card from a 270H it is obvious why it makes sense if you look at the rate of valve lift between base and max lift.

Crane makes Mercury's cams, I have personally found out, the hard way, they know what works and what doesn't, in a marine engine. Up moderate, down easy, modest lift, medium dual springs. Sustained high rpm makes heat everwhere, valves, lobes, pistons, valve springs, guides, etc, all bad news.
These max. lift cams work great on drag race engines, for seconds not minutes.