250-500cc WR300 Head Coolant Leak

So, a few weeks ago on a ride my bike overheated pretty bad. I thought it was due to the thermostat locking up, but I had no proof of that. I gutted the t-stat and filled the coolant back up, rode it around the yard real quick, and then it's just been sitting in the garage for a few weeks.

Today I pulled the pipe so I could drop the subframe and remove the shock, and at least a cup of brand new coolant came out of the pipe/motor. Uh-oh!

So I pulled the head (super easy on these bikes):





When I took the head off, the inner o-ring wasn't seated nicely like that, it was all crooked; you can see what I think is exhaust blowby on the right side of the picture. Based on the drip marks on the cylinder and the puddle of coolant in the power valve port, it sure looks like the coolant was coming in from the top of the cylinder, past that o-ring. Everything cleaned up nicely and the cylinder looks really good, so I just did my best to get the coolant out and coated everything with a light oil to prevent corrosion.

Questions:

Is there a way to drain the crankcase? I am worried there might be some coolant down there that made it past the rings.

I measured the squish, since I already had the head off. It measured about 0.075" (~2mm) right at the edge of the cylinder. Should I send the head off to be machined?

Any other recommendations for things I should look for, to make sure everything is happy when I button it back up with new o-rings?

first question, when you said overheated did it burp out the coolant, was it low when you gutted the thermostat, did you know if it was low before it overheated
the reason I am going here is did it leak and overheated because of this or did it overheat due to a low coolant level
the seal could have been wrong since assembly and would have caused this but not sure it was in your case

After it overheated, the cooling system was very low. However, I don't know if it overheated because it was low, or if it was low because it overheated.

This must not have been an issue on original assembly, because I've never had a cup of coolant in the pipe after 1 week. It must have happened because of the overheated.

Looked at your pump yet might have an impeller issue?
On the o-ring it could have been slightly out of place looking for the right time, not like it has a choice just like a perfect storm so to say
The possibility of the thermostat causing a heating issue and this is just the collateral damage is Lso a very likely suspect
I would review my entire cooling season
There is no way to dump the bottom end short of flipping the bike or removing the engine and flipping it
You could try compressed air to get most of it out and spay some wd40 or Lps or even alcohol the. More air to attempt to dry it
Best bet is to start it as it drys it and lubes it too

How do you guys check cylinder and head flatness on these motors?

I've done this plenty on multi-cylinder blocks, but those are a lot easier because you just slap a straightedge across the whole bank of cylinders at a few angles and look for gaps.

On the WR, the head has the inner ring protrusion that precludes just putting a straightedge across it, and on both the head and the cylinder I'm not sure where to look for gaps with a feeler gauge except along one edge at a time.

Any suggestions?

use clay, it will tell you the difference between your 2 parts

glass pane?

We use a 14" self adhesive sanding disk attached to a steel plate. I'm sure you could stick it to something you have that is flat. Lubricate with light oil and oscillate it around by hand. Badly warped heads must be turned on a lathe and compression relieved. I suggest our coolant too, it runs at super low pressures. It also doesn't contain water and won't harm your engine oil components like water will do.

Everything checked out with a straightedge, both directly across the cylinder and at about 60 degree angles working my way around the circle.

To doublecheck, I am going to do a couple checks with plastigauge to confirm that the gap is consistentwhen the head is shimmed up a known amount.

One challenge is the "sealing ring" on the head means you can't just measure directly across (or just lap it flat). It needs to be turned, which requires careful fixturing.

If my plastigauge measurements check out, I am going to replace the o-rings and reassembly. Tinken, I will look into your coolant, sounds like a good idea.