I am asking this question on behalf of my son who has just recently purchased a "H" reg Yamaha DT 125, the previous owner of this bike had it standing for some months, my son only picked it up last Saturday, he discovered it was losing water, checked the water pump and found that it has ceased up! What would cause this and does it mean he would have to change the head gasket too? Many thanks.

rust, 2 dissimalar metals, no additive in the water.

I'd check al the galleries and get it flushed out, personally I'd strip the engine

Thank you