Is the coolant system...

... a sealed environment, i.e no overflow?
Thanks.

overflow is a pipe on top of the header but it is a sealed system and only there as a presure releif i beleave, if it wert a sealed system it would boil

ok ta, so if you overfill it will spill out?
Not clear on the level that water should be...

it does state on the top (cold level) with an arrow going down which is basicly the join all the way round the top half of the bottle. and the therory is it will spill out but i wouldnt over fill just incase for instance the cap is nakered and dont relef the exess pressure then could start blowing other thing instead