Article on The Internet Mini Encyclopædia
The mini in (older) standard trim loses excess coolant to the road down an overflow tube. With a hot running engine over successive runs, this can lead to a reduction in the radiator coolant level and eventually overheating.
To prevent a lowering of the coolant level an expansion tank is needed. This holds and then allows the return of coolant forced out by expansion of the coolant as it heats up.
This is no smart thing for a mini - they are used on all modern cars and modern minis and have common elements:
Fitting an expansion tank to the standard mini rad and setup has two problems:
There exist a number of donor expansion tanks which may be suitable for retro-fitting to a mini. Those using the same radiator cap size offer the easiest route, as the blanking cap from the donor radiator can be put on the mini rad and the mini rad pressure cap fitted on the donor expansion tank.
CAUTION: donor cooling systems may be higher pressure than the minis so check the cap pressure before using it on a mini!
Austin Americas, 1100, 1300 and Allegros provide an ideal
source for an expansion tank. Other cars may have tanks suitable but the mini
overflow pipe is small in diameter; many modern expansion tanks are plumbed
using multiple large diameter hoses as they form a continuous part of the
coolant flow loop. Likewise, some cars do not have a radiator cap at all and
just use the expansion tank to fill the radiator.
Check the relative position of the highest point of the donors cooling system; Imagine the expansion tank in the same relative position to the top of the mini radiator as a guide but, since the effect of gravity on the expansion system will be negligible, the top of the inner wing (in the case of drop front minis) or the scuttle panel will provide a suitable location.
If, on setting up and on the first run, water can be seen to be moving into the expansion tank and then NOT going back in when the engine is cooling, suspect the overflow pipe or the neck on the radiator is not sealed. This can be brazed round the edges if required.
Failure of the expansion tank due to "suck back" can be caused by:
Expansion tanks commonly have a level marked to ensure correct operation. This can be used as a start point for use with the mini. A 1300 Allegro tank works fine at the marked level.
TIP use transparent fuel line as the linking hose then you can SEE the tank is working - expansion tanks commonly become opaque with time.
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