overpressure

Have just done a full rebuild of my 05 yz450f and have noticed that the radiator pressurizes and dumps a lot of coolant after about 20 mins of quite hard riding the cooland was no longer visible in the tank any ideas?

Head gasket?

Have just done a full rebuild of my 05 yz450f and have noticed that the radiator pressurizes and dumps a lot of coolant after about 20 mins of quite hard riding the cooland was no longer visible in the tank any ideas?

I have the same problem after a rebuild. I'm suspecting the head gasket too. With the cap off and running higher rpms i start to see air bubbles.:smirk:

You can test for the presence of combustion gases in the cooling system. Most good shops should be able to do this, or an inexpensive kit can be bought to do it with.

How did you gentlemen torque your head bolts?

Did you have the cylinder and head mating surfaces checked for flatness?

As Gray said, the torque method for the Yamaha's is rather unusual but it works. If you didn't follow the proper torque method I would suspect that is your problem.

I didnt put the head back on but Will it be ok to loosen off the retourque using yamahas method

... the torque method for the Yamaha's is rather unusual but it works.
It's still a bit unusual in the motorcycling world , but it's been evolving into the standard in the automotive world for the last twenty years or so.
I didnt put the head back on but Will it be ok to loosen off the retourque using yamahas method
I might work, but if the gasket's been leaking, there's less chance of it. May be worth trying.

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