| Every blown head gasket that i have experienced has pumped most of the water out of the cooling system, through the overflow.
Diesels are the most spectacular - its a bit like mt. etna erupting under the bonnet, sending orrible brown stuff all over the place.
I shouldn't say this, but i've not had one go on a bike before, i imagine the result is similar though.
Look for the easy fix first, you'll know when it has really gone. | 
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