125-200cc Coolant in Bottom End--Head O-Rings?

The title kind of says it all, but I'll give some background.

2012 WR 125, 20 hours on chassis from new, 40 total hours on Walt's 165 top end that I transferred from my last bike.

The bike was running beatifully at the last hare scramble of the year. I had to make an extended pit stop to fix a flat. After sitting for probably 15 minutes, I could not restart the bike. I pulled the plug and it was fouled with coolant. I cleaned the plug off as much as I could and put it back in, but no dice. Compression seems good.

I finally got around to pulling the motor apart. No signs of cracks in the cylinder wall or in the ports adjacent to water passages. The inner o-ring has some damage and looks like it was burned up a bit.





It's hard to see, but definite damage.

The base gasket looked fine. Good enough that I considered reusing it (I HATE scraping gaskets!)



So I thought, great! New o-rings and I'll be back in business. But once I started to clean up the piston for reassembly, I realized there was a LOT of coolant in the bottom end. I flipped the bike over and probably 1/3-1/2 cup poured out.



Is the tiny damage from that inner o-ring the cause of all that coolant in the bottom end? It seems a stretch that it could have just emptied that much in there. My main concern is whether it's possible for coolant to leak into the bottom end from below the base gasket, and whether I need to split cases to check. Which I totally don't want to do on my 20-hour-old cases.

Help?

Pretty sure it can. They are so reliable I've never had one split! But, those two holes at the front of the base gasket are coolant pasages, and one is in either side of the case halfs. So i'm pretty sure that coolant passes between the cases in the front. I guess it could have come from either location, or the head o-ring.

Dave,

It is hard for me to believe that you had that much coolant pass the head gasket/head surfaces into the crank case. This should be a pretty tight seal even without the o-ring. Check your head to make sure that it isn't warped. Blown head o-rings tend to show up as gasses in the coolant rather than the other way. I have had a couple of the higher compression heads burn o-rings through with subsequant lack of compression and gasses in the coolant. There is very little distance between the combustion chamber edge and the o-ring in these kits and when running higher compression it can cause some o-ring issues like what you are seeing. Certainly not the majority but enough so that I no longer send out any kits with higher compression.

If you shut it off hot and it was sitting with pressure in the radiator it would cause lots of coolant to migrate directly in to the cylinder. This usually never happens because normally you would be riding and as Walt said the combustion would be forcing to coolant the other way (bubbles in the coolant). You just happened to shut it off at the exact moment when the "O" ring failed and the radiators were still full causing your cyl to get a big coolant enema. Really you are kind of lucky because if you had continued riding you would not have known of a problem until the bike overheated and most likely fried the top end.

That's what I was thinking. When I stopped, the cooling system was pressurized and the piston must have stopped in a position with the intake ports uncovered, letting the coolant leak into the bottom end. I will do new o rings once Walt checks the head out (thanks, Walt!) and that should get me rolling again.

Did that work? It could also be the seal on the water bump

When the water pump seal fails, it lets coolant into the gearbox not the bottom end of the engine.

I think lankydoug hit the nail on the head. That is about the only scenario that would get that much coolant into the bottom end without blowing the rod bearing as the first indication. Cam.

Dave's head was slightly warped directly under the water jacket out let to the rads. Only in one small arc where the o-ring then was failing. I took .007" off the deck to get through the warp and then another .003" to make sure the complete surface was pure. Not really sure what was going on to warp only that area.