What is the correct flow on expansion tank please..

[mrzigazaga]

Hi peeps.. After replacing AAD mounting plate fixed hoses back and the expansion tank for rad sprung a leak.. Can you tell me which pipe goes where.. I know the top overflow is for atmosphere.. Bottom is out..The 15mm was connected to the AAD mounting plate but has a copper reducer in end bit of hose and it seems that is in the wrong place as one end is 15mm and other is 10mm (monting plate).. The 10mm outlet/Inlet on expansion tank is going to the rad neck overflow,,There are one twin outlet(15mm,10mm) on mounting plate and one 18mm outlet(this has hose that comes from heater matrix on it)Its the top two that are a bit of a miff.. All help appreciated.. thanks

[MJG280]

There was no expansion tank in the original. You may have to make it up as you go along

[mrzigazaga]

Hi MJG.. I feel a bit of a dumbo cos i just found the diagram in manual... I have found an ally one for £50 on ebay.. its the only one with 4 outlets.. looks nice as its mirror polished..(loads of cleaning req)..I think it holds 0.5ltr im hoping thats enough..I do intend repairing the old one just in case... Many thanks.. Mark

[honestjohntoo]

mrzigazaga said:

I think it holds 0.5ltr i'm hoping thats enough

I dont think that would be sufficient. I believe the original RV8 expansion tank, as fitted, say, to the SD1 was about 1.5 litres. It would normally be topped up to the maximum line, about half full.

One would need to examine the main purpose of the tank, which is to allow the excess fluid created by the full expansion of the coolant in the RV8 to flow into the expansion tank WITHOUT ANY fluid being expelled when the pressure builds up higher than the pressure cap specification, typically 15 psi.

Only air should be displace thro the pressure cap so when the engine subsequently cools down the same amount of air is sucked back in.

Normally there is a max level for the expansion tank regarding topping up. If the system is overfilled, that would be OK, but some fluid will be expelled as well as any available air.

Then when the system cools down only air is sucked back in and the coolant level will then be correct. But those calculations would have been made for an original size expansion tank not a tiny one.

If you fit a tank with only 0.5 litres capacity then fluid is always going to be expelled and, of course it can never be recovered, so the system will require continual topping up.

Mind you, if for some reason all the air cannot be purged from the system the above explanation goes out of the window as fluid will be expelled instead of air.

One way out of that problem is to fit a none pressurised catch tank with the overflow pipe routed into it. Then when the system cools down any expelled fluid could be automatically recovered. One could even use such an innovation to provide for automatic topping up. seemples.

Edited by honestjohntoo on Friday 10th September 19:04

[mrzigazaga]

Thanks honestjohntoo.. I have now realised that the bottom outlet on the new unit is only 8mm whereas exsisting is 18mm.. I think that tells me ..no good.. Will try to fix old one its only a small pinhole leak on neck of inlet (plastic) Have required some cargo quiksteel plastic epoxy putty.. Its resistant up to 500f and besides its only a teeny weeny hole.. Save myself 50 notes... cheers