Ok...no cheating. Scroll down, look at the photo and determine which of my exhaust manifold gaskets failed.
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This gasket was brand new. When I ordered them, some of them came with the reinforcing ring around the center hole, and some did not. The one that failed did not have the reinforcing ring, but then again neither did the one on the right. The one on the left has the ring. The one that failed separated into two, that's not a shadow on the right side of it, that's the other half peeled away.

It failed immediately since when I started it after putting the manifolds back on (and making other changes) it sounded like a clacking under the valve cover. Somehow I thought an exhaust manifold leak would sound very different, but when I took it by the local Jag shop they both instantly said "manifold leak". So I went home, read that if you blow air into the tailpipe you can just listen/feel for the leak...and it didn't take long to find it that way.

Took about 5 hours to fix...I'm just glad it was the front manifold and not the back one.

Live and learn...

John
1987 XJ-S V12, 62000 miles (and sounding MUCH better now).