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MG Midget and Sprite Technical - Crossflow radiator expansion tank

IOW Frogeye with 1275 engine and crossflow radiator.

At the moment the expansion tank is located behind the heater just infront of the scuttle. I want to move it to beside the radiator where I think I have room with the forward hinged bonnet. Can anyone tell me how high it should be? Is there a critical height for the cap or outflow pipe? A picture would also be really useful.

best regards,
J.N. Williams

Damn just put the midget away otherwise I'd have snapped it for you. It's about the same height as the rad, certainly not higher. Don't think the height is critical as being a sealed system the rad draws the water through as and when required rather than relying on a gravity feed.
There's a picture on page 68 in the Haynes manual of a late 1275/1500 location if you have a copy.
Here's a pic if you haven't the Haynes. The owner has very kindly painted the cap green - how thoughtful!

Jeremy Tickle

IIRC the height is not critical as the tube in the expansion tank goes down near the bottom of the tank so when the system cools coolant is pulled/pushed back into the main radiator.
David Billington

Thanks Jeremy and David. Very helpful.
regards, Nick
J.N. Williams

Ermm,, if that is a pressure cap and the place where you fill the system surely it should be at the highest point in the system to prevent air locks ?
Andy Phillips (frankenfrog)

you may well be right but when refilling the coolant I use the separate plug on top of the radiator, which IIRC is the same height or slightly higher than the expansion tank as that overfills slightly before the rad does.
Jeremy Tickle

Topping up coolant via the expansion tank is the correct procedure on the rare occasions it's necessary. It should be about half full.
As the system is sealed, there should be no problems with causing air locks unless a lot of coolant has been lost and the expansion tank drained. If that is the case there is likely a problem elsewhere.
Peter Blockley

The highest point is the top connection at the heater core. I always do the final top-up on mine by disconnecting that and using a funnel into the heater ppe. I have also used a piece of heater pipe with an air-bleed nipple built in (ex scrapyard Peugeot). Simply filling at the expansion bottle doesn't immediately fill the whole system as its too low. If using Peter's advice, you need the car to go through several heating / cooling cycles to dispel any air and to suck up coolant from the expansion bottle as it cools, on order to get a properly filled system.

Alternatively some recycled domestic plastic bottles use a filler neck which is just the right size to push firmly into the expansion bottle neck. Cut the bottom off the bottle and use it as a funnel sealed to the expansion bottle, so you can fill it to above the highest point in the system, bleeding off any air as you do so. The first time the car heats up it will blow out any excess coolant and that's job done.


I agree with you entirely and I also do the final top up from the heater pipe when filling the system from empty.

The point I was making was that for occasional topping up, the expansion bottle will be fine so long as it still has some coolant in it, because there is nowhere for air to get in. Unless there's already an air lock or a problem elsewhere, of course.
Peter Blockley

This thread was discussed between 30/04/2016 and 02/05/2016

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