Hey Guys and girls.

A quick question.

So.... a plus motor, 1275. This should have a 13 lb long necked radiator cap yeah?

If you have a long necked 7 lb cap will it get to temperature and then over flow?
DC

No.. a lower pressure cap will just allow the coolant to boil at a lower temperature - should it get that hot. A zero pressure cap would allow the coolant (assuming its pure water) at 100C.

Anti -freeze and a pressurised cap just raise the boiling point.

Cheers, Ian

Probably , in Australia , yep

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

Sort-of on topic:

Are there special caps for overflow bottles? I was thinking of making my own overflow bottle, but I wasn't sure that the normal cap would let any coolant back in.

I've got an overflow header tank from a moke on my clubbie., works a treat.You put your pressure cap (13lb) on the overflow tank and a flat radiator cap on the radiator.Coolant expands into the header tank and is pressurised then returns to radiator when cool

Oh, ok, that sounds interesting. I was thinking of having the overflow unpressurised, so the radiator drops as usual (pushing the coolant into the overflow), but when it cools back down instead of sucking in air from around the cap, it would suck it back in from the overflow bottle. I wasn't sure if this would work without a special cap though.

I might try it my way first and see if it works with a standard cap. As nice as an authentic overflow would be, there is a certain satisfaction of trying something cheap and having it work.

I got this info from a radiator tech. I dare say you could just let the coolant expand into the overflow tank without a pressure cap trouble is though if you don't pressurise one end of the setup you could end up with the entire radiator boiling out through your header tank. You have to pressurise the system to allow the coolant to expand and open the thermostat to the block All the books talk about the pressure cap being on the expansion tank particularly the moke

In normal operation without an overflow, the excess coolant just flows out from the cap and down a tube onto the road. I was thinking about having this tube lead into an overflow bottle.

When the car cools down, it will (hopefully) suck the coolant back from the bottle into the radiator. Normally it just sucks air from around the cap, and the radiator drops a little. The problem is, on hot days it drops further than usual, and without an overflow it'll stay dropped.

If I stick a bottle at the end of the tube, the radiator's still pressurised by the normal cap, so nothing can go wrong. It's just a matter of checking if the radiator will suck up the coolant from the tube, or suck air from around the cap (like it does at the moment).

OK, it goes like this:

If you have a non-pressurised overflow, it needs to be above the level of the radiator neck so that gravity can feed the fluid back into the radiator as it cools. You need a radiator cap with a drainback valve for this to work.

If the overflow is pressurised (like mine which is an MG one I think) then you can place it anywhere. Mine is under the wheel arch, out of the way. With this type you run a pressure cap on the overflow tank, and a flat cap on the radiator. The pressure in the overflow (and the vacuum created in the radiator) forces fluid back into the radiator as it cools, and as long as there are no leaks you're fine.

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Simon

The adventures of an owner builder in the Tallarook Ranges