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Radiator cap op. pressure

Martin
Posts: 18
Joined: Sun May 07, 2017 8:47 pm
Location: Wateringbury, Kent, UK

Radiator cap op. pressure

Post by Martin »

My '67 V8 2 1/2 litre is running very well and keeps a very even temperature (and oil pressure) but I am regularly topping up the radiator with a pint or so of 50:50 anti freeze. I am not losing fluid through leaks and the level seems to settle just out of site from a visual inspection through the top of the rad. The workshop manual seems to support the current 7psi cap fitted Re engine number but is there any reason why I should not try a 9 psi cap as preferred for earlier cars? Was it a preference change for 'later' cars to go 7 psi cap or was there engineering reasons or some failures beyond 7 psi? The rad, hoses etc are all in a good state and the cooling system had a very thorough flush last year.
I have not taken the thermostat out and tested that as the car gauge gets to op. temp and then just sits there very stable. On v. hot days (last year!) in jams then the electric fan quickly brings the temp. back to normal......so I'm keen to try a 9 psi cap if no risk.
Thanks for any help, Martin (in Kent)

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John-B
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Re: Radiator cap op. pressure

Post by John-B »

Is your cap long enough to provide a pressure seal? Note that Jay Leno made the mistake of using a short cap when restoring his SP250 and the coolant boiled. See Leno's Garage and his test run at the end of the video:

tjt77
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Re: Radiator cap op. pressure

Post by tjt77 »

A std 7lb cap should keep the water in if cap is in good shape.. its wise to add an overflow/expansion tank, which creates 'sealed system' as with modern cars and allows coolant to be sucked back into radiator when it cools off..
Often when coolant is dissapearing and there are no external leaks, it is an issue with head gasket(s) due to corrosion over the years .. a pressure 'leak down test' would be wise .. the kit to do this is easy to source and inexpensive..

Chris_R
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Re: Radiator cap op. pressure

Post by Chris_R »

If you don't have an expansion tank there needs to be an air gap at the top of the radiator. If you don't have an expansion tank then what is most likely happening is that excess coolant forcing its way past the radiator cap and out of the system. If it is settling at a certain level then that is the level it is happy at. You don't need the radiator totally full, what you need is sufficient coolant to keep the engine cool and if as you say the temperature is stable then I wouldn't worry about it. Just drive the car and enjoy it. We tend to worry too much about these sorts of things when in reality there is nothing to worry about.

Martin
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Joined: Sun May 07, 2017 8:47 pm
Location: Wateringbury, Kent, UK

radiator cap/ expansion tank

Post by Martin »

Dear All
Thanks for replies.
To answer some of your obs. cap is long enough but still wondering if 9 psi would do the job, head gaskets fine, have seen posts/photos re tank but as last poster observed if all is well, and I make sure it is a constant amount i need to re-fill, it does appear to run at some 'equilibrium' level. Was a bit concerned as thinking of driving to classic Le Mans and it was always a bit dodgy in the Sunbeam Alpine at 30 c in a traffic jam! The V8 has two Kenlowes and seems to deal with everything. Thanks for replies. K r, Martin

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Jez
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Re: Radiator cap op. pressure

Post by Jez »

Running the system at 9lbs will slightly increase the boiling point of the coolant but as you're not having temperature problems then that won't make any practical difference. 9lbs pressure will also mean a higher system pressure before the system vents and may therefore increase the coolant level at which the system finds equilibrium - the level at which there is room for coolant to expand at pressure when hot. I can't see any reason why the extra stress would cause an issue - some vehicles with essentially similar technology run at 15lbs, although the increased pressures there will find any weaknesses in the system, most likely down to age.

Many owners seem comfortable so long as the level doesn't change over time and there are no signs of overheating. If the level concerns you, then the more modern expansion tank is your solution. The tank provides space for the coolant to expand into when hot, and it is then drawn back into the main system as it cools and contracts.

To make this work, you need a radiator cap with a second outer seal. This prevents air from being drawn in around the edge of the cap and therefore creates the suction to return coolant from the expansion tank. The tank itself can be pretty much anything convenient - on my Dart I use a period style 'Tudor' plastic windscreen washer bottle. The pipe from the radiator must go near to the bottom of the tank such that it is covered by the residual coolant level, but it doesn't matter if it is bottom or side entry or simply pipe pushed down through the cap. It's wise to provide an overflow from the top of the tank, although in normal operation it wouldn't be needed.

Cheers - Jez
Jez Stow
1960 Daimler Dart
and a few other toys

Martin
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Joined: Sun May 07, 2017 8:47 pm
Location: Wateringbury, Kent, UK

Re: Radiator cap op. pressure

Post by Martin »

Thank you Jez; that's a very useful post.
Kind regards, Martin

Flinty
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Re: Radiator cap op. pressure

Post by Flinty »

Jez
I am trying to locate a pressure cap with a double seal as you described in your post, Can you tell me where you obtained the cap you used.

Steve.

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Re: Radiator cap op. pressure

Post by Vortex O'Plinth »

Flinty wrote: Wed Oct 17, 2018 9:25 pm I am trying to locate a pressure cap with a double seal as you described in your post, Can you tell me where you obtained the cap you used.
There's a 7psi long reach double seal cap here.
Nick

"Don't bother with the Air & Space Museum - there's nothing to see.......".

Flinty
Posts: 90
Joined: Mon Sep 24, 2018 7:34 pm
Location: Scarborough

Re: Radiator cap op. pressure

Post by Flinty »

Thank you Nick the seller is on holiday but I have saved it for later.

Steve.

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