125-200cc 91 250 WXE Crank Case Full of antifreeze!

I will start at the beginning. This bike has been in my family from 98. It became mine about 6 years ago. I never had any problems with it. It was used as a backup bike. My godson was riding it this year and it lost compression. So when I took the head off I was amazed that the top of the piston was super clean with just a tiny bit of carbon on it. So I thought the head Oring was leaking a tiny bit and steam cleaning the piston. So I put a new piston in and new head and cylinder O rings in it. I start breaking it in over two days by running 3 minutes. Both times ran great. The third day I go to start it and I can hear water sloshing as I kick it. So I take the cylinder off again and see the crankcase is full of antifreeze! What could of cause this?? Any help would be appreciated.

a leak HAHA IM NOT FAMILIAR WITH THE COOLANT ROUTING ON THAT BIKE BUT MY FIRST GUESS IS THE BASE GASKET

I put a new base gasket in when I did the top end. When I took the cylinder off the base gasket looked fine. What I don't under stand is it ran twice fine. Then it filled it self up.....

coolant in the crankcase can enter through the cylinder, cylinder head or their gaskets. There may be a coolant passage through the case halves but that's unlikely to be the problem because it hasn't been disturbed (unless it's been corroded by poor coolant)

You mentioned it lost compression - why? There may be a clue here as to what the problem is. A head gasket leak bad enough to give a massive compression loss would have blown coolant out the radiator and left a very obvious witness mark.

You may have a porous or cracked cylinder or cylinder head.

Inspect all components carefully and thoroughly, I'm sure you'll find the problem mate.

My guess, you have a center case coolant passage leaking, something warped, cracked, rotted through, etc....

Also, had one that did that. The studs/nuts dragged as I torqued everything.
Torque wrench clicked as being like 20 foot lbs, but nut was not drawn down tight. The nut was dragging on the stud.
So make sure all your fasteners are wire wheeled, cleaned up with die or tap, etc...
I always squirt lil WD 40 on my stuff/and or anti seize, that I don't loctite, to get accurate torque readings.
For life of me couldnt figure why base gasket was leaking and nothing was warped, cracked, gouged, scratched etc...

I always scuff gasket surfaces with scotch Brite pad for grip and never use gasket glue or anything on surfaces. Then wipe all surfaces with rag and carb cleaner, to make sure nothing was oily. Look at your base gasket to make sure it wasn't sucked in or blown out a lil.

your engine is a typical LC modern engine
it uses O-rings to seal the top and hoses to transfer the coolant
the cylinder and head are the only items that see fluid
your issue is either O-ring or a casting issue of the head or cylinder

Water pump seal, sleeve spacer and small O ring.
Later George

George, that was where I was going to look into next. If the water pump O rings where leaking. If they are leaking would it also mean the main seal was leaking and the antifreeze was getting in that way?

On a 2stroke a water pump seal leak allows coolant into the gearbox not the crankcase.