It's been 4 1/2 years since this thread was originally started.

I would think Jim has everything sorted by now.

is that a light weight con-rod and piston on an OE weighted crank? --if so, that can't be good. (read: hard vibrations/casting/fatigue...)

Now thats funny, I didn't even check that

Thats a billet rod( better than stock) and a stock piston. These engines run 7000 grand all day long

Good catch SM.
4.5 years later, the reasons why any engine case would pop remains valid, though.

Could a mix of incomplete mixture(gas runoff into the case), with high compression and blowby(i.e. spark) cause this?

"better" won't matter if the crank isn't balanced for the lighter weight.

Fatigue failure is ALL about the number of cycles and load. Inducing large vibrations into a casting (esp Alum)- not helpful.

Metallurgically, the billet rod is substantially better than a cast stock rod and is actually heavier. As for your balancing concerns we are dealing with a single cylinder 4 cycle engine, vibration is simply a fact of life. Static or rotational you can only balance at one RPM setting anyway. Balancing a single does not reduce vibrational amplitude just its vector. And that is a guess.

The Animal was designed for racing RPM's, in stock rules racing there are no aftermarket parts that will substantial upset this engine.

An assembly or a casting issue would be clear early on. With the condition of the parts and the age of the engine it's evident that this is not an assembly or casting issue.

This was apparently a rare failure mode in the Animal 1st gen, so it was a design flaw in the block casting. My experience was with 4 and 5th gen engines, which had noticable structure changes in the block. There are some indications that chassis flex contributed to these failures. According to sources there has been no failures of this kind in the 4th and 5th gen's running in the stock classes.

Product improvements highlight a company actively involved in what they build. It does not represent any design flaw whatsoever. To say that a 5 or 6 year old engine without any known history failed because of a design flaw when there are tens of thousands still actively raced doesn't add up.

Any product bought used should be gone through. Would you buy a used car without having a mechanic look at it? This is a 5 year old car, the odometer was broken, not to mention a million other possible variables are in play (from chassis flex to to chain tightness).