I've got an Ali header tank for my cooling system on my ford blacktop.

To what level do people fill these to as there are no low or high markers ?

I'd go for half way to allow room for expansion.

Yeh 1/2 would be fine.

Topped up to half way

Cheers guys

From my experience it's surprising how much expansion there is of water upto 100C... I would measure the old air volume in the original expansion tank with a measuring jug and some water, (i.e. from max to over flowing!..) that would tell you the volume you need to be as original expansion space.. Then measure the replacement in the same way and mark up appropriately...

Cheers

Dan