1Moses1, what you say makes good sense, but the OP’s failure is at the turn on the runner and nowhere near a weld. In this case I don’t think we can say “That’s what headers do.” I’m pretty sure he had another issue by the looks of his pictures. Has anyone else seen something like that in the middle of the runner? Also, it’s in the outside of the bend, not where you would expect water to collect and eat through.

The op’s failure is at the thinnest part of the tubing after it’s bent. Very common. The tails are generally a thicker wall material and less tight radius bends.

your original post asked about risers not tails...

really? i have bent plenty of tubing and i cant see it would be more than a few thousands thinner.

Thanks! My Mercury stock exhaust is already back on, but just changing the exhaust manifold was not the reason for this thread.

And also the least amount of water flow

This is exactly my question and the reason for this thread. What caused this issue? I think everyone can agree on that a stock 496HO is not an extreme engine by any means which is one of the reasons that makes me woundering what is going on.

The failure is in the tubes from cyl 1-4 which have the biggest radius. From cyl 5-8 the where the radius is tightest the tubes looks like new, not a slightest sign of pitting. Can You please explain which part of the exhaust is the tail. I thought it was the part that is clamped on after the collector?

From the pictures that you have uploaded, I see minimal corrosion and a very hot section of tubing at the outside apex of a runner that most likely lost water flow. Do you have a sea strainer? It seems very possible that you picked up some small chuck of crap that blocked flow and you over heated that spot in the tube. If I was in your position I would get another set of the same headers and add a sea strainer. Stock sh!t sucks and stainless marine is just bilge clutter.

I agree with you that this is a heat issue and there is no corrosion. If it only had happen to one of the runners I would also think there was some kind of obstruction but the damage looks almost exactly the same in all runners from cyl. 1-4 while the runners from the rear cyl 5-8 looks like new and as I mentioned before, there has been no discoloration on the polished surface. I do have a sea strainer, and after taking the headers off I back flushed them and the trees without a trace of debris in them. The flow was pretty similar through all the runners if not slightly more through the damage ones.

If it is a problem with lack of water, have anyone heard if it is possible that an air pocket could have formed at the highest point where the damage are or is it just the weak point for water flow? If it is engine related, even if the 496HO is a mild engine can it due to bad injectors create a lean condition that can cause this? Lean condition due to low fuel pressure should have affected all the runners equal or?