Expansion tank, plumbing in

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:

• a blank cap is fitted to the radiator to maintain an airtight seal on the neck and the movement of coolant to/from the tank
• a tank connected to the overflow pipe from the radiator by flexible tube capable of maintaining flow under conditions of low pressure (10-20 psi) and vacuum - this tank has sufficient free space above the coolant level when cold to allow the free expansion of the coolant
• a standard pressurised cap on the expansion tank as would normally be fitted on the mini radiator to allow the coolant system to pressurise when hot (and the coolant has expanded) and maintain a temperature somewhere in excess of 100c without boiling out.

Fitting an expansion tank to the standard mini rad and setup has two problems:

• the radiator cap as fitted has a sprung seal inside- it will let water out but not back in
• the overflow pipe may be only a push fit and not brazed in allowing water to go out but allowing air to suck back in when the coolant in the system contracts

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:

• Blown head gaskets - these lead to water being blown out of the engine, breaking the continuous column of water from rad to tank
• Air leaks in the coolant system - sucking in preferentially, though these should show on hot running as water leaks
• Removal of the radiator cap when the coolant is hot releasing the pressure in the system, and "breaking" the line of coolant between radiator and tank - as the water cools it will contract and the vacuum created will suck out from the expansion tank
• Overfilling of the expansion tank - This can stop expansion of the coolant as water is relativly incompressible and the tank relies on it being relativly empty when cold to work!

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.