Header tank cap

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Header tank cap

Post by adamgeering »

Dear all,

i have one of Gerry's header tanks for my ace replica and have a 2.5 straight six triumph engine in with with an MG V8 radiator. I am not sure what cap i need for the tank. I think i need a 7 Lb standard cap but CBS and Rimmer Bros think i need to see what the original Ace had. I cant see what difference that makes as neither the cooling system or engine were in the original Ace. Any help with what type of cap i need and where i can get one would be very much appreciated.

Thank you all

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Re: Header tank cap

Post by peterc »

Hi Adam, There are a couple of things to be considered.
Firstly the pressure rating effects the boiling point of the water. The higher the pressure the higher the temperature the system will withstand before boiling.
What pressure rating did a Triumph engine use. I think quite a few of the Hawk Cobras use 15lb.

Another issue which several people ( including myself) fell foul of is ensuring that the top of the header tank is slightly higher than the very top of the entire cooling system. Basically the older classic style cooling system needs around 1.25" of clearance in the header tank for the water to expand into. If you fill the header tank to the brim it will spew out coolant when hot leaving it around the 1.25" below the neck when cold. If you top it up it will merely repeat the action on the following trip.

My problem was that the heater was higher than I realised. I resolved this by adding a MG Metro over flow bottle and connecting the bottom of the overflow tank to the outlet on the Hawk header tank neck.
There are debates within the club about where to have the pressure cap and where to have a fixed plain cap.
Personally I have a fixed cap on the header tank allowing the water to flow both into and return from the over flow tank. I have a pressure cap on the overflow tank.
Some pressure caps have a dual flow facility which allows water to flow back in past the cap. In that case you can have the pressure cap on the header tank and a fixed cap on a non pressurised overflow.
I hope that assists.
Peter C
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Re: Header tank cap

Post by clive »

I have never seen the point of a separate over flow tank as well as the header tank as the header tank acts as an expansion tank. When I fill my system, I fill it through the top heater hose raised above the heater unit, which allows for any trapped air to escape. I then make sure that the coolant flows up through the bottom hose of the radiator and through the header tank until it flows out of the heater. I then run the engine until warm and let it cool. Any excess expanded coolant will come out of the overflow but there is no need to top it up as it will leave room for the coolant the next time you run the engine.
One thing to check when selecting the pressure cap is the length of the spring and sealing rubber. Initially mine was too short by a couple of mm so it didn't seal correctly with the flange on the header tank. Off hand my cap is 14lb on my Ford 302 and was bought from my local motor factor. No need to go to any specialist supplier.
Cheers, Clive.

(If I'm not here I'm in my workshop or on the golf course!)
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