Hi,
Does anyone know the internal diameter of the bottom coolant hose on the MK1?
I intend to fit a proper header tank with a t-piece in the bottom hose to a small translucent plastic tank on the bulkhead as high as possible; also thought to connect a t-piece into the coolant hose going to the throttle body and run a pipe into the top of said header tank so that the system will self bleed. Can anyone see any problems with this idea?

bump

Seems like a lot of effort just to avoid having to bleed the system. Since the pump, cooling channels and radiator were designed for pressurised closed system use, I wouldn't run the risk. Bleeding is a simple procedure, and if bleeding needs doing often enough to warrant this modification, then something is wrong with your cooling system.

Thanks Mike but it will still be a pressurised system but with more capacity and that much easier to check the coolant level the self bleeding is just a bonus; its my daughter's car now and she is not good at checking levels but she does not ignore warning lights and I can fit one to detect a low level in the header tank; I think the lack of a header tank contributes to the frequent HGF's; have had 3 thus far (albeit at 110,000, 150,000 & 205,000 miles......)