Rad Cap Pressure Qs

[BobbyG]

Hi guys - I have a flathead v8 with a new aluminium radiator - I have been running a 4lb cap - would I be better to run a higher lb cap - say 15lb.

I have noticed that after a couple of miles when I turn engine off it purges a pint or so from overflow pipe.

thanks

Bob

[flathead 53]

try to find out what the mfg of the radiator recomends

[BobbyG]

No name on it - fitted by previous owner.

[19Fordy]

15 psi is pretty high. Make sure your radiator doesn't split at the seams and bulge at the top tank. Also, check to see that your hoses don't split. I would suggest a 7 lb. cap with a max. of 10.

[FlatheadTed]

Even a 15 lb cap will likely purge some water in a modern system it gets sucked back in when it cools .you need a two way valve cap to do that .

[Old Henry]

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If you fill the radiator to the top the mere expansion of the water from the heat will push some water past any radiator cap. After the first time filling to the top then blowing some out there should be an inch or two at the top of the radiator to allow for future expansion without pushing any water past the cap. The only way to prevent that air gap at the top of the radiator is to get some kind of coolant recovery system that lets the water expand out into a tank when hot then get sucked back into the radiator when cool just like all modern cooling systems. I don't think it's necessary. Mine and most others' have always worked fine with the air gap in the top of the radiator.

[flathead 53]

I have a new radiator in my 53 ,not aluminum, I run a 7lb thats what they recomend ,using 160 deg stats no overheating

[ford3]

its not original , but run a recovery tank, it will suck the over flow water back into the radiator, ive always been leery of high pressure caps on the flathead, not sure the seals on the water pumps can handle that kind of pressure very long

[Henry/Kokomo]

15psi = flirting with disaster. There are better ways - several enumerated here - to solve your issues.

[FlatheadTed]

Yeah stay with the 4lb

[ken ct]

Originally Posted by BobbyG

Hi guys - I have a flathead v8 with a new aluminium radiator - I have been running a 4lb cap - would I be better to run a higher lb cap - say 15lb.

I have noticed that after a couple of miles when I turn engine off it purges a pint or so from overflow pipe.

thanks

Bob

The rad. might be able to with/stand 15# but what about the fact that the engine and all other components are 70 yr old type. Hose connections,head gaskets. These were not desightened for 15 # of pressure. ken ct OMO.

[ken ct]

Originally Posted by FlatheadTed

Yeah stay with the 4lb

I agree Ted, hes filling it too much. ken

[Cecil/WV]

I would fill it to the top and let it run with the cap off and see if there are any internal problems causing pressure to push the water out.

[Tom Walker]

I don't know if you ve covered it previously on the Barn, but on another of my cars I make the original cap seal completely, and run the overflow tube to a small tank down behind the radiator cowl. This tank has a pressure cap, only 5-7lbs say, and the tube from the rad must run to the bottom of the pressure tank. From cold, you run a small amount of water in the bottom of the overflow tank, and the radiator full. As the engine warms up, expansion fills the overflow tank. If it has sufficient capacity, it will level off, and as the engine cools, water returns to the rad.

I find this works very well, and just a small amount of pressure makes a big difference to keeping the water in the system and raising the boiling point as well.

Downside is the extra tank, which is not an original "look"

Cheers, Tom.

[BobbyG]

Thanks all - I am sticking with the 4lb cap for now - leaving the top of the rad empty and seeing how she goes with that set up.

Thanks all --- again

[FlatheadTed]

Your right Ken ,Like Tom the little over flow tank will do it .

[G.M.]

With a 4 LB WORKING pressure cap or Skips 3 LB on older radiators with no provision for a pressure cap I fill all of my radiators and have hundreds if not thousands of other old Ford owners filling their radiators up to the top of the filler neck. My cars never lose any coolant and stay filled for well over 6 months in hot weather driving. The pressure cap must seat down in the bottom of the filler and the cap on the older type radiator must have a good gasket to seal it. To cool these engines properly the water or coolant needs to be forced through the radiator. There is to much resistance in the radiator tubes, that is why water comes out the over flow if the temperature is less than 210. Much over 210 and the pressure generated by the hot water opens the pressure cap and allows water to run out the overflow. G.M.

[GEOFFNZ]

In our experience with a competition flathead the pump seals will not stand a 15lb cap.13lb seems to be the max.they will stand.Also to suck water back there bust be a very good seal around the top of the cap or it will just suck in air. Cheers Geoff NZ.