Photos...
 

Here are photos showing the cracked head; I didn’t take photos of the timing chain housing or melted chain ramp.

As mentioned, there were cracks through the top of the head and up the sides to the intake, although the corner of the head was still connected and held in place by the head stud. However, one little rap and the entire corner of the head broke off. The portion of the head near the intake had not cracked all the way through and was physically snapped when rapping on it; the portion nearer the "notch" was obviously cracked all the way through and had been melted/carbonized and you can see where it had been vibrating against itself.

The biggest issue is trying to decipher why it happened. Most likely suspects are overheating due to a lean running condition or perhaps pre-detonation. Once we pull the cylinders we’ll take a good close look at all of the rings to see if any are broken (as occurred back in ’04). There are some significantly more knowledgeable people than me frequenting this site; I’d be interested in anyone else’s ideas.

It's been 15k track miles since my total rebuild, so from here it is top-end rebuild, including rings, head studs, valve fitment, Aasco valve spring checking, and injector cleaning/flow-matching; then we'll dyno it to check for anything out of the ordinary. If nothing, then it's track time!

JB Weld.

Since it will be apart, put some water cooled, 4 valve per cylinder heads on it Eric. With the twin plug design you already have, I bet you would like the results.

Cool. Almost a clean path of least resistance. Around the valve seat and through one of the intake port stud holes. I wonder if the stud was the propagator? Cranked in too tight or through a bunch of existing crud stuck in the threads. Might answer why the crack wandered around the valve seat but did not include the spark plug hole. That little tear in the side of the sheet of paper. Just add heat and stress over many cycles.

Eric: lots more knowledgeable folks than me hanging out on this board, but here's a long shot: I had the 6.5 hp Briggs & Stratton engine on my Billy Goat lawn vacuum go kaboom. Actually, it blew a head gasket, burned a valve, popped a valve seat out, etc. Needs a new engine.

Mechanic said cause was a mouse nest, hidden by the engine cover, which caused the engine to overheat. He found the dead (slightly fried) mouse, plus the nest materials near the head. He said he sees a lot of this on lawnmowers, snowblowers, etc., and I know from personal experience that rodents will nest in auto engine compartments.

Doubt seriously that this caused your problem, but should be a cautionary note to p-car owners who store their cars over long periods of time to look for rodent infestation. Sorry about your engine.

..overheat tends to drop seats out too. Would be kind of funny, in a not funny way, for Eric's motor have gone kaboom from being messy.

i had a riding mower in a shed and after starting it up for the first time in the spring found the air driven governor was not helping to hold a steady RPM. I then noticed a mouse running out with a pink baby in her mouth. I shut the motor off and watched as she made 4 or 5 more trips in and then back out with another baby and under a nearby pile of firewood.
The air cooled motor was packed with mouse nest made from a nearby towel carried in thread by thread.

Like rain on your wedding day or a free ride when you've already paid.

This is what I was thinking as well.

wow