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what caused this engine failure?

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I've seen similar damage from a seized piston at top of stroke, skirt broke and piston wedged in up tup, the pin yanks right out, and the rod proceeds to bash the hell out of everything

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If I was guessing, I'd say extremely seized piston, water ingestion, or possibly over-revs. Do you know what caused it, or are you looking for answers?

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im looking for answers, so heat seize? conrod broke out, and all hell broke loose?

there was oil in the head, cant see if there was oil in the crank.

Im going to assume overrevved,

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Thats not an 08, right? This is an older motor. I see an external oil line so its pre 07. It looks like the cause is the oiling issue on the 03-06 450s. Not enough splash oil to lubricate the cylinder and wrist pin because the sump is pumped so low. Siezed the piston and broke the rod. This issue was fixed in '07 by installing an oil squirt nozzle. They have rev limiters, so its not that.

Thats a bummer.

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It's pretty obvious, really. Look at the piston.

We have one pin boss in place, one torn off. The small end is till on the rod, and there's that cute little wear mark outside of the clip groove on the broken pin boss...

You dropped a circlip, most likely because it wasn't installed correctly. The pin slides out until there's only one side of the pin boss holding it, and the combination of the doubled up load and the rocking strain breaks the piston.

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It looks like the cause is the oiling issue on the 03-06 450s.
There is no such issue with the YZ450 motorcycle engines. That was a problem that the quad engines had for a time. When the oil spray nozzle was added to the '07 YZ/WR engine, it was done for piston crown cooling, but people wrongly concluded it was for the oiling problem the quad had.

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from another qaud site:

"04 and 05's used a flat circlip too hold the gudgeon pin in place, 06 and onwards use a round circlip the reason is that apparently the gudgeon pin would push againest the flat circlip in the older models and eventually unseat the circlip, eventually the gudgeon pin would move out of place and cause this sort of damage. the round circlip in the newer models is actually forced deeper into the retaining grove when pressure is applied too it.

You are lucky your conrod didnt do more damage."

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If the piston had seized as one theory hear is offering, both pin bosses would have to have been broken off in order to disconnect the rod without bending the end of it over and/or turning the piston sideways in the bore. The fact that only onside is broken is a giveaway pointing directly to the clip.

There are two mistakes that can be made when installing the flat, "TruArc" type snap rings:

  1. All such rings have a right and wrong side. One side will be very flat clear to the edge, with sharp, square corners, and the other will have a more rounded, stamped look. The flatter side must always be placed so that it bears the holding load, and in the case of the piston pin, that means the rings must be installed flat side out, against the groove.
  2. The rings should be oriented with the gap between the ends facing straight up. At extremes of RPM, it is actually possible for the inertia generated by changing directions to flex the clip enough to lift free of the groove and potentially dislodge it. Orienting the clip as above minimizes the effect as much as possible.

It's also possible for the pin to rotate the clip into a disadvantageous position (2), but if the clip is installed right side out (1), that should be much less likely.

A third, less common, but serious mistake is using the wrong clip for the piston. Never use a wire ring in a piston cut for snap rings, or vice-versa.

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