We were having overheating issues with the race mini at the 24 hour race in California. We were chalking it up to the 110 degree weather, but realized after driving half the day by the thermostat, that our tube had pulled out of our over flow catch can ( yep same Dasani bottle as bills car, it's a bit of a trademark...)

Without the radiator having othe ability to expand and contract the water we would overheat and have to spend valuable time in the pits. An eagle eyed crew member fixed the tube and ran it to the bottom of the overflow bottle and that was the end of our overheating for the weekend. We ran the car to 6500 rpm for 18 more hours and the temp was pinned at 180 degrees. 

I am a firm believer in the overflow bottle. We run a 12lb cap

If you're chucking out that much coolant, my first qustion would be are you using water with a dash of antifreeze in it or the pre-bottled crap ?

50/50 mix is rubbish at cooling...

My observations of most modern systems (in various non-Minis) are that the manufacturers use a pressure cap on the radiator with a non pressurised catch tank with a return line.  That way the radiator is always topped off with coolant.

I use an old 500 ml plastic brake fluid container to replicate this sytem.  The catch tank sits neatly on the cross member just in front of the wiper motor.

Cheers, Ian

If it pushes coolant out when it gets hot, that coolant is useless.  Coolant drawn back on cool down serves no purpose.

With an expansion tank, the rad will stay completely filled with coolant, and any steam created in the system can get to the expansion tank and condense.  This is where you improve the system.

Not sure I agree with the last statement. If the water pushes out of the system and is drawn back when cool that stops fluid loss. Depending on where the pressure caps is changes things but the system still works. I run my cap on the expansion tank and it has an over flow tube to the air. I can't help but think volume of fluid helps. Steve (CTR)

An expansion tank is part of the pressurized system.  What you have fitted is an overflow tank.  The activity you see with fluid moving out of and back into the rad is of no value in terms of improving the car's performance.  You can accomplish as much simply by not over filling the radiator.

Thanks for the input guys.  I ended up using Dimitris' suggestion and bought an expansion tank off a Polaris snowmobile (they run $15-$40 on eBay).  It bolted nicely to the wing behind the radiator and the tank fills from the bottom.  I filled the radiator up to the overflow tube and ran the car around town for half an hour.  About one half to three quarters of a pint of coolant expanded into the tank, but when the engine had cooled completely, all the cooltant in the expansion tank had been "sucked" back into the radiator with no leaks anywhere!  Here's a photo of the tank installed.  Thanks again.

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One could solder the overflow spigot on rads like mine where its a loose fit.

All non mini 1300 powered cars had a proper expansion tank in the system, as did minis built in the colonies.  I use a Volvo 120/140 type vertical cannister tank and neat bracket.

This means that the radiator is always filled with coolant.  Letting that expansion occur in the radiator reduces the mass of coolant the system has to work with.  Note that medium duty engine makerss call these de-gas tanks:  They give the system a place to deal with steam that might be created at hot spots in the head.

I installed a burp tank up under the LHS wing.  You can see it here.  It seems to work fine with the line from the radiator connecting to the bottom fitting.  I bought it on one of the online racing shops.

Every Mini i have owned has burped "excess" fluid out when filled to the top, i always leave the level about 1 to 1.5 inches down from the neck and have no problems although a catch tank is a good idea.

two fingers thick above the core is pretty normal to me!

Good point CupCake.  I guess I was thinking of a "catch basin" tank more than an "expansion tank" in terms of keeping any radiator overflow from dribbling on to my garage floor.  Seems every time I top off the radiator, the coolant expands when the engine heats up and goes out the overflow tube.  Then when it cools back down, the coolant level in the radiator is back down to just above the core.  Maybe I should just leave it at that level and not even mess with any kind of overflow tank?

FB

An expansion tank system consists the pressure is maintained by a pressure cap on the expansion tank. The cap on the rad is a a solid cap is no pressure valve. The existing overflow tube on a Mini is just to direct any blow-by down to the ground in the event of over heating. On my Mini the spigot for the blow-by tube is loose ie not sealed in the rad fill tube. If yours is the same an expansion tank will not work because the coolant will be open to atmospheric pressure.

check this thread here (mid page) for a nice solution from an ATV

//minimania.com/msgThread/115675/1/1/1961_Build-Restoration

I use the MGB or MG1100 brass ones. Not cheap but work well. Steve (CTR)

What do you guys use for a radiator expansion tank?  I don't see any in the catalog (except for the MINIs and fuel injected Minis).   My Mini (with a stock 998) came with an old 1.5 pint Dasani plastic water bottle.  The bottle is tucked down sort of beside/behind the raidator and held in place by only friction, but considering the heat and vibrations, I don't think it's going to last very long.

FB