Coolant pressurising -leaks where?

[jools]

We've been chasing this prob for a while. The RZ250 is pressurising the overflow bottle when it gets warm.
We've done the obvious things... hoses, thermostat, radiator cap.
There's no oil in the coolant and no coolant in the oil. We do see bubbling in the radiator running with the cap off.

Where else could it be getting pressurised? Waterpump gasket? Ready to pull the head (if we can get a new gasket!), but any ideas where else the prob could be?
-Jools

[Greasemeup]

Rad cap might be seized or wrong temperature for AU conditions?

Check pump prop/impeller?

[jools]

Well we can discount the radiator cap because both the old one and the new one are working fine.
It's pumping plenty of fluid so you'd have to think the pump/impeller is good.

[Greasemeup]

That doesn't leave many options, next is a blockage, price of mineral build up broken off and lodged somewhere perhaps?

[jools]

Yup, don't think it a blockage problem though, - I was really thorough when I flushed it all out and cleaned it up when I did the top end re-co.

The coolant is flowing freely, but it's pressurising, -blowing coolant out of the overflow bottle. We'll try it again with all the hose joints etc smeared with grease to rule out air leaks there.

Anywhere else air could be getting in?

[jools]

Yup, don't think it a blockage problem though, - I was really thorough when I flushed it all out and cleaned it up when I did the top end re-co.

The coolant is flowing freely, but it's pressurising, -blowing coolant out of the overflow bottle. We'll try it again with all the hose joints etc smeared with grease to rule out air leaks there.

Anywhere else air could be getting in?

[Greasemeup]

That was my next thought, an air lock somewhere, after that I am out of ideas.. Where's that shrug emoticon?

[hybrid]

My first guess would be head gasket, even though there is no oil in the fluid.

Although if it was a head gasket, it would probably pressurise when cold too if you give it some stick.

[jools]

You may well be right. It does seem to pressurise even though it might not be really hot. Now I've hit a snag in that it's pretty hard to find head gaskets. (plenty for the 350 but the 250 ones are more scarce).

One of the things that occurred to me when I reassembled it, is that because of the separate barrels and one-peice head setup, if the barrels aren't perfectly aligned (say, because of mismatched base gaskets or incorrect torquing on the base nuts), it may be that the head may not sit perfectly parallel on both barrels. I did do a check for flatness and height of the barrels, they seemed ok, but I wasn't fastidious about this. I almost feel as though the head should be torqued down to the barrels BEFORE torquing the barrels down to the crankcase if poss. (So any slight inaccuracy would occur at the cylinder bases not the head joint).

[rocet23]

test the thermostate,old and new one.if you drill 2 3mm holes in the themostate it will help with taking pressure out off the cooling system.this mod will allow water flow when the thermostate is closed.if it dosent presurise when riding ,not in traffic,but gets hot in traffic it can be a waterflow problem,water pump faulty,blocked radiator or thermostate.if it works out to be a head gasket i allways get the radiator serviced.my radiator bloke removes the tanks and cleans with core buy putting rods through the core.

[jools]

Ah, that's an interesting tip - drill the thermostat, maybe worth a try.

It pressurises not only in traffic or when hot, seems to be mostly after a highish speed run. (That right S ?)
Probably will pull the head, then we'll know for sure if it's that. (Hopefully the repro gasket I'm going to order will be ok).

[hybrid]

It's been a while since I have had the head off one of these, but surely the 350 gasket would work OK? There should be enough meat for it to seal I would have thought.
Downside is the combustion space will increase I guess.

[jools]

Yes possibly 350 gasket would be fine, just that the sealing ring will be a little further out... can't see that making much diff.

[rzipper]

We've found a stripped cylinder head sleeve nut (no. so this could well be the cause.
Has anybody in OZ got good spare head sleeve nut(s) handy?
We have an inlaws biker gang ride soon, and it'll be touch and go whether we'd get a replacement from the UK in time.

[jools]

Quick update: We helicoiled the stripped headbolt thread, fitted a new 250 head gasket, torqued it all down and it's survived a couple of good test runs with no leakage.
Great. So even though there was no sign of any oil in the coolant, the right hand pot was pressurising the cooling system, caused by one bolt not tightening because of the stripped thread. Job done.