2012 with a blown case seal, need advice

I have a 2012 wr450 supermoto. It has 1100 miles on it. Today while riding it blew the seal right at the bottom rear where the two case halves are mated together. The silicon was blown out and it dumped its entire oil contents. It looks like it happened a mile from where I stopped, and it was running and not making any abnormal noises when I stopped it. It looks like it dumped about half it's oil contents in the driveway, lost the rest while riding. I wasn't beating on it while any of this happened. I've done top ends on dirt bikes before but never split a case in half. How much of a project am I looking at here? Am I going to be in over my head and need to bring it to a shop, or is this something the average joe can do. I already have the shop manual and am reading up on what a project it looks to entail. 

I would try to determine the root cause of the problem before dumping a bunch of time and or money into splitting the cases.  Sounds like a plugged crank case breather or something.  There shouldn't be enough pressure in the crank case to blow out the case halves.  Are the halves gasketed or just Yamabond'd?

It looks like the halves are just yamabonded or whatever goop they drop in there. no gasket. Im looking at tearig it all the way down to check the crank and stuff and make sure I din't cause any damage when it ran low on oil. 

Not normal.


The cases are a precision machined pair. Yamabond is applied (a tiny bead) to seal. Sometimes, a tiny bit ozzes out and dries on the surface. Owners have been known to 'pick at it' and pull the sealant out. I have never seen it blow out, esp. on a 4s. Just no pressure to do it. I have seen the seal fail on a 2S but there were other issues excaberating it.

If the breather becomes obstructed, I've seen YZ450's famously blow out the decomp bore plug in the '03-'05 engines, and/or the half moons at he cam cover. If the pressure has nowhere to escape to, it accumulates and will do something like this. 


But there are two things odd about that in this case, starting with the fact that the breather on the WR's are tucked away a bit better and routed into the air box, so the opportunity to pinch it off under a bent clip or whatever is less.  The other thing is that the sealant has supposedly let go first, which I haven't ever seen happen as a result.


If this were mine, I'd clean things up as well as possible, get all the skid plates and stuff out of the way, then verify that is actually the source of the leak.


But to answer one of your questions, whether you're over your head or not depends on what level your head is at.  The bottom of most Yamaha 450's comes apart pretty nicely without a lot of fuss, and even though the book does show some special tools involved, beyond a flywheel puller, you shouldn't really need them. 

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