Mondial radiator cap, which one is correct?

I'd say run it. It looks like the rubber on the newer one matches with the actual contact patch of the old one. The upper rubber, right against the cap is merely additional protection from getting a bit of fluid up there. Mostly any fluid that gets past the first seal under pressure will leak out the drain line. That looks like an old cap you were running there.

The one I installed on my car looks much like your replacement. My old one wasn't that old and looked similar as well.

No technical experience in this area so you can take it for what it's worth.

And on .9 vs 1.1 I have a opinion counter to everyone else. It's an old car with many old connections. I don't want to increase the pressure if I don't have to. In a newer car, higher pressure allows for higher operating temps and more efficiency. IMO in an old car I'd rather keep a weak link in the system in the form of a lower pressure cap. I'm sure in some cases it keeps some of these cars from overheating.
This is my uneducated opinion. And I seem to recall people smarter than me suggesting the higher pressure cap. My car runs great with the .9 so I'm holding until I have reason to change.

PS. I'm not opposed to being wrong so if someone has a good argument for the higher pressure that I can understand I'm happy to change my opinon.

 

Hi Peter,
I went to the garage to take some pics for you. Mine is .9 bar square radiator cap with rubber seals.
I believe the square cap is more of an oem accurate part. I bought mine off ebay supplier over a year ago for about $30 USD.
And yes my new cap is the exact same style with rubber seals as my old rusty one.

http://www.ebay.com/itm/FERRARI-RADIATOR-CAP-9-BAR-101499-/360492170107?pt=Motors_Car_Truck_Parts_Accessories&hash=item53ef01fb7b&vxp=mtr
Image Unavailable, Please Login
Image Unavailable, Please Login
Image Unavailable, Please Login

 

The wrong cap would have the wrong pressure as air/coolant would leak out. I would try it and monitor results or side affects noticed like coolant spilling out, a drop in reserve tank level, or temp gauge rising. Yes a bad radiator cap can make your car overheat. It can also spill coolant on the ground. Whenever you see a puddle of coolant on the ground, especially if the car is warm or running, the radiator cap is the first thing you suspect.

The way a radiator works, it's full of coolant. As the engine gets hot, the coolant expands. It's supposed to be under pressure, because pressurized coolant can get hotter (like water in a pressure cooker), so it can carry away more heat, so your radiator works better. But it can only be under so much pressure before things begin to burst. So the radiator cap has a spring in it. When the pressure gets above a certain threshold, the cap backs off and allows some coolant to flow to the overflow reservoir. When the engine cools off, it sucks the coolant back from the reservoir.

The cap has a number on it, either pounds or 'bar' (atmospheric pressure), and that's the amount of pressure it holds. If the spring gets weak, or the gasket wears out, it won't hold that pressure and you'll either not have as much cooling capacity or else it will spill coolant on the ground as it expands, instead of it going to the reservoir. You can buy another one cheap enough. It should have the same number on it for the same pressure.

 

And Mike said it without saying it. Being pressurized it can go above regular boiling point without boiling. If the cars go to boiling point without the cap the coolant can boil creating air pockets in the engine and the air does not carry the heat away allowing further overheating.

I think the second seal is to keep coolant from spitting out around the cap top onto the expansion tank when the system goes over .9 bar. My guess is that it's good to have but not crucial to the running of these cars. We don't technically have overflow tanks, our overflow goes to the ground, but we do have that expansion tank.

I don't think I took pictures of mine so I don't know whether mine has one seal or two.

You might call Eurospares and see if they have any knowledge on this. I did just find a picture of my old cap and it was the more square variety that you started with. I think my newer one is like the one you now have. Mine I obtained by looking in the cross reference thread and getting one at a local auto parts store.

 

Hi Peter, you may well find that many of early Fiat UK cars have the same cap, and all still available at the right price, if you dont find one then let me know and I'll see what I can do.

Nino.

 

Hello
I replaced my rad cap last year with the updated 1.1 bar cap and have not had any coolant leaking on the ground since.

 

The square cap .9 bar is the same as used on Fiat/Bertone X1/9

Alfa Romeo, Fiat and Lancia Coolant Reservoirs and Caps