Yamaha 426 milky oil - help

Hi guys, thought I'd make a new thread for this to try get abit more help before I start stripping it down

Just got the bike and having bad luck with it, supposedly had a weisco high comp piston fitted a few months ago.

Starts ok and rides fine, feels really fast and idles ok.

But the oil is fully milky coloured, drained it and refilled it and it happens again straight away. No leaking coolant or dripping anywhere though

So I reckon it must be head gasket?

Thanks for any more help

Yes, head gasket is a possible

So is a cracked head, or freeze (welch) plug (not sure if the WR has these)


Is your coolant level going down?


Another possible is short trips doesn't get the engine hot enough to get rid of the condensation that gets trapped in the engine

Thanks graham

If it was cracked head wouldn't performance be affected?

Bin told condensation but it's warm weather here and new oil goes fully milky within minutes of it running so kinda guessed it wasn't that

A lot of coolant must be getting through

And coolant doesn't appear to go down but iv not ridden it for more that few minutes

Thanks again

My WR suffered from milky residue in the breather pipe from the starter clutch housing, when I first had it, coz I was riding it on the road

They have no thermostats so it was over cooling


If yours is going milky within minutes, then its something more serious

There could be a crack from the water jacket to the where the cams are, and hence it would run OK but loose lots of coolant

If the coolant has frozen at anypoint in it like then it may have cracked the head/cylinder somewhere.


It's got to be coming from somewhere, so if you have a crack then you will loose coolant fast


As its recently had the head off, then head gasket is first place to check

Usually leaking head gaskets result in gas being forced into the coolant (under the force of the burning petrol) resulting in it blowing coolant into the overflow bottle, coz if its leaking enough to allow coolant into the engine, then the pressure of combustion will blow gas into it much faster


Yes I know its been warm here in the UK recently!

Thanks for your help buddy

Just bin for a ride on it, no coolants gone down in the rad?! But the oils gone milky again. Rides fine and bloody fast

What the hell

Hi guys, took the water pump apart today



The lip on the shaft is quiet deep, can really feel it. Would this be making my oil go milky so quick?

Cheers guys

Actually that lips meant to be there isn't it

The step belongs there, but the grroove where the coolant seal runs is a wee bit deep looking. (see attached pic) Replace the shaft, both seals, and the bearing. 


Pay attention to the seal orientation; it looks backward to most people.  Read: http://www.thumpertalk.com/topic/841710-04-450-coolant-leak/


BUT.... No, that will not turn your oil milky unless someone or something has plugged off the weep hole under the pump.  The space between the seals is vented down and out into the atmosphere specifically so that that won't happen.  Possible points of coolant entering oil are, head gasket, base gasket/O-ring, the O-ring at the right crankcase cover/crankcase joint behind the water pump, or a crack or hole in any of the water carrying sections of the RH cover, cylinder, or head.  There's a hex head plug in the cam box area of the head that can also leak, but it won't unless someone has disturbed it.


Awsum thanks grayracer. Il get the waterpump done and out the way

So there's another seal on the crankcase iv already taken off then that I should check?

It's apparently had a high comp piston fitted recently so it could be anything once I get the head off

Thanks again

And to turn the oil milky within minutes I guess must mean it's a big leak somewhere

And to turn the oil milky within minutes I guess must mean it's a big leak somewhere


Yes, a big leak which should also be confirmed with the coolant level dropping in the Radiator - do not judge it by the level in the overflow bottle under the rear mudguard if you have some coolant in this. You must check the coolant level by removing the rad cap


As grayracer says, if the water pump seal leaks, you get a dribble out of the vent hole which drops onto the ground, under the engine on the right hand side

There normally start leaking when the engine is first started and then stop as the engine gets warm - a kind of early warning system to let you know the seal needs replacing


Check that the hole is still present, and the PO hasn't blocked it up - its not unheard of for people to think this drain hole is the problem and block it instead of the seal

picture here http://www.thumpertalk.com/topic/1027758-water-pump-question/#entry10808681

Edited by GuyGraham

Rite iv realised the engines had some big trouble before and PO has bodged it and sold it on


Guessing a valve has snapped and caused damage round where the bucket sits? Would this be leaking water ?

Piston and cylinder look ok so I'm guessing a reconditioned head is the way to go?

Thanks guys

The tip of the center intake stem does look so happy, either.  I'd say your guess was correct.


As to the water seal on the crankcase cover, it's an O-ring, about 16-18mm in diameter, directly behind (Looking from the right toward the cylinder) the water pump.  #13 in the image below:



Thanks gray racer


Would this crack be leaking water quick?

Is it a crack or a score in the plating

If its a crack through to the water jacket then yes its enough to leak water a good rate

It's a crack about 1.5 inches long through to the water side

What would of caused that? Over heating ?

Hard to say with any certainity


Yes overheating, previous blown up, metal fatigue or the water froze due to lack of anti freeze (most likely). At least you found your problem


Don't despair - Langcourt can repair it http://www.langcourt.com/page16.html


There are only two places in the UK who actually do cylinder replating. Langcourt is one, and Aptec is the other

Everybody else sends it to one of these two places

Had my WR cylinder done by Langcourt over the last winter - top job

Edited by GuyGraham

Thanks a lot

Iv ordered a cheap second had cylinder from a breakers which is apparently in good condition, just to get it running and make sure everything is ok

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