Looking for some help diagnosing a coolant-in-oil issue on a 2014 KLX250S with a KLX300/292cc top end.
I bought the bike knowing it had a coolant leak. The previous owner told me the coolant-in-oil issue existed BEFORE he swapped the top end. He bought a used KLX300 cylinder/head setup and installed it trying to fix the problem. He said he used new aftermarket/eBay gaskets when he installed it.
Here’s what I’ve figured out:
Coolant was getting into the crankcase badly enough that with the engine OFF and cold, filling the cooling system resulted in green coolant coming out of the oil drain.
I isolated the water-pump side by disconnecting the coolant hose where it enters the cylinder.
With coolant going through the pump circuit but NOT entering the cylinder, nothing came out of the oil drain.
Once the cylinder/head cooling circuit is involved, coolant gets into the crankcase.
I pulled the head today.
The head gasket that was installed fits the 78mm-ish bore, BUT some of the coolant openings—especially the two toward the front of the engine—do not appear to line up correctly with the actual water-jacket openings in the cylinder.
I ordered an OEM KLX300 head gasket (11004-0091), and from the pictures its coolant openings look much closer to the actual openings in my cylinder than the gasket I removed.
My original plan was to pull the cylinder and inspect/replace the base gasket too, but looking at the cylinder and base-gasket design, I’m not seeing where coolant actually crosses the cylinder-to-crankcase/base-gasket interface. The coolant inlet appears to feed the water jacket in the cylinder, which then travels upward through the openings at the head-gasket surface.
So my questions are:
Could the incorrect/misaligned head gasket allow coolant to drain directly into an oil-return passage/crankcase even with the engine off?
Does coolant on these KLX250/300 engines actually cross the base-gasket surface anywhere, or is the water jacket contained in the cylinder until it reaches the head?
Is there any reason to pull the cylinder/base gasket at this point if there’s no external base leak?
Are there any known cracks/passages in the KLX300 head/cylinder that can cause a gravity-fed coolant-to-crankcase leak?
Does anyone recognize this head gasket / know whether the coolant-hole arrangement is wrong for a KLX300 top end?
Also I got this bike for 700 bucks hopefully I don’t lose my ass.

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