250-500cc Dodgy head orings?

Keep losin coolant(100ml or less generally but today was bout 300ml!). Swapped hoses out recently to chinga silicone gplus brand, which seem pretty good n cheap price, but was losin it pre hose swap so not hoses. Oem hoses were leakin in coupla spots. Coolant still clean can't see any oil/discolouration from exhaûst gases. Did change coolant brands recently but both were/are ethyl glycol same concentration etc. rad cap seems fine. No holes in rads it's comin out overflow. Only thing I can think is inner oring has let go? Anything I missed?

It is summer here n hot n deep dry sand but it's never lost it before n I'd expect it to run ok without losin much if any at all

If it's blowing out of the overflow it could be the cap. How about the pump, have you got flow? Feel the hoses as its running, give it a few blips, you should feel a flow if you squeeze one of the hose's. It doesnt take much to change the head o rings, I'd go with them first.

Yep sorry got good flow. Orings seemed obvious culprit to me blowin exhaust/gas into cooling system displacing coolant. I might order new orings n a cap too if it's not a total rip off. How would cap cause coolant loss-no pressure retention? Can't see any leaks at all jus comin out overflow.

Do the 125 rad caps fit the 300? Could swap caps if they do. Not at home to check

Head o rings is possible have you had the head off at all?
Its easy to pinch the things upon re torquing the head on.
I know you and where you ride so no point asking if your rads are clean as they will be.

If your loosing up to 300ml thats half the system then your bike would be followed by white smoke, has your mate noticed anything?
Rad cap probably does interchange get wifey to try it for you.
If your spring in rads gone weak then it will open up sooner and fart your coolent out the overflow.
Not in one go but over the ride bits here n there when the engines hot.

Yeh it's hot n dry here ATM. Yesterday was out across the road in the sandbowl with mate n bro havin uphill(few hundred metres round power pole then down-2 laps per race) sand dune drags. Deep black sand. Didn't even think bout the 165. Even the 300 struggled as did Bros 500exc. In sayin that we are muppet girly sand riders who don't know how to/to scared to hold it on like you should. Was low-mid 30s so fairly warm n ringing bikes neck for coupla hours so she got warm. Rad cap sounds feasible. It's the original so will see if it is interchangeable with 165. Think 165 is 1.2bar where as 300 is 1.4bar. Make any diff? Orings are over 30 hours old with no visible leaks around head, white smoke or coolant loss. Did have 165 orings leak after similar hours so possible I guess.

Ps the old 300 won 3 outta 5 races n I was givin away 15kg to mate on his berg 350 n a full bike length or 2 as a handicap(keeps em interested if they think they're a chance!). Bro similar weight but 500 didn't seem to get power to the ground was jus trench diggin. Once the WR was up on top of the sand it was jus a matter of feedin gears n holdin it on. Was pullin away from the 350. I got caught in fencing wire for one race n stalled n got cleaned out on the hairpin round the power pole on the other(filthy take out move ala Stewart/reedy). Great fun n I made sure I reminded em of how the old shitter 2t with hundreds of hours on it mopped the floor with their fancy new age thumpers. Made me smile all afternoon

Would only blow white smoke if coolant being burnt but can't get in can it? Normally gases push it out as it cooling system fills with exhaust gas n push coolant out overflow due to expansion?

Well if the o rings are gone im guessing it can suck and blow coolant through the same bit.
1.6-1.2 bar is roughly 5.6psi differemt blow off, try it see if it makes any difference you could have a bad rad cap seal.
You show em whos boss shawry!

Had that with my cr250 for a long time. PITA. It is the head O-rings. If you have time and patience a hydraulics shop can find you better O-rings. Upon installation pop them in the freezer for a while- they will shrink just a wee bit and might seal when torqued down- took me a fair bit of coolant to get it.- finally\fingers crossed

Il try the rad cap first but yeh I'm thinkin it's the orings. Easy enough to do on these bikes tank don't even have to come off don't think(maybe to get at head stay bolts?). Don't spose anyone knows the sizes of the orings as I'm at work n we have hydraulic hose presses, seals, orings etc.(underground mining side of things). Could possibly source them here for free, free I tells ya!!! Cheers

The rad caps should be the same size.

Fortunately the o-rings are cheap! I used to have an RMX & it was something stupid like $80 over here for a head gasket! It certainly made inspecting the top end pricey!

I would of pulled it off intact just done a base gasket... Unless those were £££ too then i would of sold it asap

Lol! I wish it was that easy! The head casting had a chamber for a thermostat as a part of it which made it impossible to remove the cylinder without removing the head first! Or at least I could never figure out how to. Unless of course, you wanted to take the entire engine out of the frame! Such a pain! ........I sold it & bought a WR300!

FYI rad caps are different. 300 has a bigger plunger bit than the 125 but 125 was longer(I think it was that way around). The 125 will go on the 300 but it compresses the plunger. 300 won't go on 125 at all.

On a side note I got the shift shaft seal in(after buggerin around with the clutch cover tryin to get it back on-forgot Pv has to go in cover then install) n have new head orings to replace so see if that cures prob. After that new rad cap!

Sweet hope you didn't damage cover or pv mech.

Nah I'm not that rough juicy! Didn't go wailing on it with a mallet or anything jus couldn't work out why it wouldn't go on when it went in first time last time. Was looking at Pv mech tryin to think then after bout tenth attempt remembered Pv mech sits in case first then put it on. Was being very careful takin it off n on not to damage shift seal. Hopefully cures oil leak. Im at work for coupla weeks so will see if any has dripped out when I get home. New head orings n she should be good as gold!

Kalgoorlie gold?
Phalangies crossed its all tip top A1 clubbing jam fur for you to thrape about like a mentallist!

Post me some of the gear you're on juicy!

Im clean baby just british! You met me you should know.