Help...YZ450 with coolant in the oil

I am stumped. I recently purchased a 2008 YZ450 that the seller claimed had a completely rebuilt motor. I test rode it and power felt strong and it appeared to have zero issues. However, when I got it home, I changed the oil and realized that the oil was full of coolant. My first thought was waterpump. I changed the seals, but realized in the process that coolant and oil couldn't mix without it leaking out the weep hole. I then proceeded to tear down the motor. I split the cases and everything visually looked good. Except the new motor wasn't really new at all. (The seller is an A-hole and karma is going to destroy his life.) I took the cylinder and head to the machine shop and had them check for warping. The head was toast so I bought a brand new head complete with valves. I put everything back together with the new head and started her up. Everything seemed fine, so I decided to drain the oil to see if I fixed the problem. F my life... there was still coolant in the oil. I am not really sure what to do at this point. Any ideas? I've searched the forums but found nothing that I didn't already check.

The water pump cannot put coolant into the oil unless someone has plugged the escape port that is drilled under the pump in the crankcase cover. There's a gap between the oil seal and the coolant seal that vents to the outside specifically so as to prevent that from happening.

There are a limited number of places coolant can cross into the crankcase oil:

  • The O-ring at the joint between the crankcase cover and the crankcase.
  • The base gasket
  • The head gasket
  • Loose or missing jacket plug at the center of the head in the cam box (hex socket plug).
  • Cracked or porous head, cylinder, crankcase, or crankcase cover

Thanks grey! I will double check the plug in the head. Maybe the new one didn't have one in it. I replaced all the other gaskets and the o ring. hopefully that solves it.

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