After fitting an expansion tank to the standard mini radiator, is it necessary to fit a different radiator cap, or will the original cap allow water to be drawn back into the radiator as it cools down?

Yes there’s a specific cap that allows the coolant to be drawn back into the radiator.
https://minisport.com.au/mini-moke-radi ... -type-13lb


Obviously there’s no big problem if you don’t fit it, just the recovery tank will be a one way event

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Thanks Timmy.
If I don't fit the correct cap then there wouldn't have been much point fitting the expansion tank either. It was a tight fit!

Timmy,
Supplementary question: cap in the Mini Sport link you sent is for a short neck tank and the minis were all long neck tanks.
We have already found a cap for a recovery system - meant to be long neck but obviously is not - it doesn't even fit a short neck tank properly.

Hi Bill,
I think there are a few things worth checking and confirming the type of tank you have - this is my understanding after various bits of Googling. An expansion tank is part of the pressurised system, an overflow tank is not.

If using an expansion tank as per the Moke then you need a blanking cap on the radiator and normal cap on the expansion tank.

If using an overflow tank (as I did) you need a recovery cap on the radiator and air venting on the the overflow tank.
viewtopic.php?f=3&t=99842&p=1061256#p1060118

Cheers, Paul.

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1970 Morris Mini K

Bill - mine is a recent Minispares radiator and I bought that exact cap and it works for me. It’s a CPC brand cap

You could also try the local parts shop - they might have a match in their stock.

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Yep, your radiator shop will be able to supply... That way you don't take the risk of getting the wrong one... (or paying the usual Mini supplement

"....An expansion tank is part of the pressurised system, an overflow tank is not... " An explanation as to "so what" would be really useful... and appreciated. All the modern cars I'm familiar with use the overflow system....

Cheers, Ian

In case you want to do a bit of research this is the CPC website. CPC is Australian and supplied many auto manufacturers, and no doubt still do, so expect a quality item.

http://www.cpcauto.com.au/

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

Thanks Paul. This is exactly what I am starting to realise as the difference in the 2 systems.

Referring to your photo of your overflow tank setup, you have an alloy radiator which will use a short neck cap. Can you tell me if your radiator cap has a seal under the top of the cap, to seal against the top of the filler neck, as well as a double seal at the end of the spring section?
Thanks.

An explanation of expansion versus overflow on this website. Whether it's an accurate description of our situation I don't know.

https://blog.championcooling.com/2018/0 ... sion-tank/

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

The Morris 1100 and some later Mokes used an “expansion tank” which is under pressure. The radiator gets the blank cap and the pressure relief cap is on the tank. These are a good way to bring the fill point up higher in case the radiator fill point isn’t the highest point. These will still spit out a bit of coolant if you fill them to the top (as they work just as an addition to the radiator). The expansion tank needs to be a high pressure rated unit

The recovery or overflow tank takes the excess coolant out when the pressure rises in the radiator, then when it cools and the the pressure drops it draws in the fluid from the tank. I have one on my car, it means there’s a reduced chance of corrosion as the system is always full of coolant and not much air

Blank
https://minisport.com.au/prod1161.htm
Normal
https://minisport.com.au/prod590.htm
Recovery
https://minisport.com.au/mini-moke-radi ... -type-13lb

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Thanks Doug, I still don't know what that tells me.... Is having the system isolated from atmospheric pressure good or bad ...or doesn't matter??

Maybe the bit of extra water and surface area of the expansion tank provides a fraction extra cooling? But then an overflow tank ensures the radiator is always full... and is essentially free.

Cheers, Ian

Thanks winabbey. I'm getting the idea now.
I think I can fit a standard mini cap with an extra rubber seal under the top of the cap to prevent air entering while the system cools and pulls coolant in from the overflow bottle.

Didn't realise this was going to turn into one of those rabbit holes.

The recovery cap has a separate little piece in the middle of the underside of the cap. It’s spring loaded and allows the coolant back in when the system cools and the pressure drops. I’m pretty sure you need the specific recovery cap

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Hi Bill,
Yep mine has a seal under the cap.
It is also a CPC cap, though a smaller diameter than the standard mini one to suit the non-std radiator. I got mine from Minisplus in Bayswater but cannot see it on their website. It has 'recovery' stamped on top.

Cheers, Paul.

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1970 Morris Mini K