250-500cc Coolant loss?

Have got a 2012 WR300 with 137hrs on it,the problem I have is the last couple of rides it has lost coolant,first time I had to put 300-400mls in the second time probably 150-300mls.
Have checked gearbox oil,hose clamps all good.Still has thermostat in and was wondering whether that might be the issue.Funny thing even on the hottest days it has never boiled and here in Queensland it gets pretty stinkin' hot.Still on original piston & rings J D jet kit running pretty sweet
Any thoughts
Cheers

not in gearbox, not boiling over its gotta be going somewhere! not burning it up(bad sickly smell n white smoke when running) through leaking inner head oring? not weeping from head due to dodgy outer head oring? rads haven't split(mine both did)? no weeping hoses or cap? water pump not leaking(oring)? im out of ideas. 137 hours wouldn't be surprised if head orings gone. time for a top end anyway id say Ran. maybe check the small hose that runs over the top of the frame near the headstem(in front of tank) that joins rads together. it rubs & could be leaking

Thanks for your thoughts shawbagga,checked that little hose between the rads and has rubbed but dosen't seem to be leaking,don,t think it's the inner head o ring as I would have thought you would see bubbles in the coolant with it running and rad cap off,no leaks on outside of head,still think might have something to do with that thermostat?
Probably pull the top off and throw a new piston at it and remove thermostat.

How low is the level? Even with or just covering the radiator fins cold is normal, if you fill it full it will puke more than that out of the overflow.

I bought a used '10 WR300 and had the same issue. Every ride, I would consume coolant. Come to find out the inner head o-ring was blown because the previous owner stripped out two head bolts. Of course the 2 that weren't possible to be repaired for what the cost of a new cylinder was.. $500 later, all is well..

Also if you can get your hands on a waterless coolant. These coolants operate at very low pressures 0-2psi and to not push through gaskets and seals as easily.

Coolant level is well below the rad fins so something is up, will have to dig deeper.
thanks for your reply's

It's gotta' be going someplace. I'd rig up a temp recovery tank on the overflow tube.

Yeh not a bad idea will think about rigging some thing up.Will prob. do top end first anyway. Cheers

Replace the radiator cap with higher rated unit.

Paw Paw