Good picture Dave, my rad is about the same I would think.
I have had a good think about it, and I don't have a lot of room for a header tank, and I don't have a flip bonnet either.
I figure out that with the bleed valve in the thermostat housing, if I needed to fill the complete cooling system from scratch, I would fill using the current rad cap, which would give me around 90%+ of the water required, and then replace the cap and top the rest up, (heater hose mainly) using the Heath-Robinson
bleed valve come filler-cap I am currently fabricating!
The system still has the safety valve in the current cap, which will allow water to escape into the overflow bottle if required to do so.
I have now finished and fitted the alternative to a header tank,
Pics:
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