Mag side meltdown

[aaronfinke18]

3 days ago I picked up a 2016 axys 800 with 5000km on it. It had a full engine rebuild with only 11 hours on it (crank, cylinders, pistons bearings ext.) I took it out for a first ride and was only pulling 7400 rpm took it home replaced the worn belt, new clutch weights and new primary spring. I took it out the next day and was getting 7700 and after half an hour of riding the top end went in it. I’m not overly experienced but to me looks like it leaned out and melted down but looking for input. This will be the second mag side piston failure in 15 engine hours.
Im thinking next steps are sending the injectors to get cleaned and flow tested and
Checking intake boot for any cracks that would cause it to lean out.
I’ve also been told it could be bottom end bearing going and sending shrapnel up into the chamber but that seems unlikely to me as it’s a brand new crank and bearings. The crank feels fine spins freely and has no play.
Any input is greatly appreciated

 

[aaronfinke18]

Both times out it was also bogging hard at wot

 

[BeartoothBaron]

Ouch! Looks like a locating pin failure - the piston ring caught the port and broke the crown of the piston. It could be related to a lean condition, although usually detonation bad enough to take something down that quick will trigger det warnings. What pistons were used? Was the block new, re-plated, or was it not re-done after the first piston went down? It could be there's a bad injector or air leak causing it to go lean and that took down the cylinder to begin with. Could also be the PO ran it well past the point where new pistons were due, had one go down, and didn't fix it right. Unless they gave you a pretty good run-down and knew what they were talking about, I'd say you have to cover your bases - you can't just go with one or the other assumption. Very unlikely the damage came from somewhere other than the piston, BTW. If something in the crank went first, it'd be obvious.

One thing you didn't mention was the crank seal: I'd replace those (obviously suspecting the mag side, but both while you're in there). Check the boots like you said, clean and test the injectors like you said, and also check fuel pressures as soon as you get it going. Could be fuel filter, fuel pump, or something electrical hurting fuel pressure; mine had intermittent fuel pressure issues that may have been the cause of a blown piston a couple years back (although it manifested as a locating pin issue; ring spun and caused similar damage). I'd go through Millennium for a rebuilt block: they may be able to exchange it and get one to you quick, and they do good work. Then, OE pistons are probably worth it, although some people report good luck with aftermarket stuff (both standard replacements and the "fix kits" with taller pistons).

Anyway, it's a really crummy way to start the season, that's for sure. I was surprised how functional my sled was with a blown piston: you knew something was wrong, but it made it five miles or so out, still started right up and went into reverse, etc. So maybe the RPM issues were fuel pressure or an injector or such, or maybe it was just from the piston going down. Like I said, cover you bases. You may not find a smoking gun, but if you eliminate any potential fueling issues and air leaks, then fix the mechanical stuff right, it should be good for years more riding.