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-   1958 To 1960 Squarebirds - General Technical Discussion (http://squarebirds.org/vbulletin/forumdisplay.php?f=14)
-   -   Radiator cap pressure rating??? (http://squarebirds.org/vbulletin/showthread.php?t=5913)

trim code 76 01-23-2009 02:07 PM

Radiator cap pressure rating???
OK guys, easy one(?) I have looked all over my books and manuals and can not turn up what the stock rated radiator cap's pressure rating is? Is it 14lbs????? You would think this was in the owners manual or shop manual but I can not find it!! As always you guys are the greatest.

bcomo 01-23-2009 02:43 PM

I have a 13 PSI cap on my 60.

I can't find it in the 60 shop manual either.

YellowRose 01-23-2009 02:46 PM

Radiator cap pressure rating???
I did a search on "radiator cap" on our forume and here is what Alexander had to say about this on 11/19/2007 & 11/20/2007.

12 to 15 lb is correct for the Squarebirds. A 13 lb is available. The original cap is B8-8100-A.

There is no point deduction for the AC cap in all classes in BTCI and ITC as that is the correct replacement.
1959 Hard Top
1960 Golde Top

In addition, there is a post that shows a picture of an AC 15# RC-15 cap. Alexander also said this..

Autolite caps from 1967 to 1980's are acceptable replacements.

If you click on the link provided in that post, you will find that there is this. AC also has a 13# RC-6 model available for those who want a 13lb cap.

I have a non-AC 14lb cap on Yellow Rose

JohnG 01-23-2009 04:34 PM

The lower the pressure the better.... less stress on that cheesy expansion tank on the top! Make an overflow bottle for inside the engine compartment (for non-show times) and catch anything that comes out so you're not dumping toxic stuff into the street.


FeFranco 01-24-2009 11:59 PM


Originally Posted by JohnG (Post 28051)
The lower the pressure the better.... less stress on that cheesy expansion tank on the top! Make an overflow bottle for inside the engine compartment (for non-show times) and catch anything that comes out so you're not dumping toxic stuff into the street.


Well put and this is what I found out. My expansion tank was leaking at the seam so I brought it to the radiator shop for repairs. The repairman told me that he would recommend a 10lb cap because of the typical temperature fluctuations in the radiator. Mine will easily see 210 on the gauge at a stop light in the summer, but climb back down to 180 when I get moving (i have a gauge). Sure, it will spit it out, but if you have an overflow it will eventually suck it back in. I HAD a 13lb cap.

trim code 76 01-25-2009 02:38 AM

Thanks guys!! Knew I would get the right answer from you.

tbirds8 01-26-2009 07:22 PM

I'm running a 7lb. cap on my 66. Freeze plugs are a little weak and what else I don't know. I'm sure I'll find out soon enough.

dgs 01-27-2009 12:26 AM

I had a 6 or 7 lb cap and it wouldn't keep the fluid in. At temp it's just spit it out the overflow. I switched to a 13 lb and mostly cured my problems. Of course, I found a weak lower rad hose, freeze plug and now either a leaky heater core or temp control valve. Oh well.

bcomo 01-27-2009 09:50 AM

I wanted to add something about the purpose of a radiator cap.

The purpose of a radiator cap is to raise the boiling point of the water/antifreeze mix.

The normal boiling point of water is 212 degrees Fahrenheit (F).

Typical engine operating temperature after the themostat opens (with good air flow) is about 200 degrees F.

You can see that there is only a small difference between the two.

A radiator cap increases the boiling point of water by raising the atmospheric pressure (pressurizing) the system. When the water gets hot, it expands, but the radiator cap keeps the water pressurized by not allowing it to escape from the system untill it reaches the psi rating of the cap.

A 15 psi cap will raise the boiling point of the water by about 45 Degrees F. So now the boiling point of the water in the system becomes 212 F + 45 F = 257 F

So, the lower the psi rating of the cap, the earlier the water will boil, and the engine will overheat at a lower temperature. A lower psi cap will definitely keep the system pressure lower, but in stop and go traffic or high abient temperature, the engine will overheat faster and blow water from the overflow.

YellowRose 01-27-2009 11:45 AM

Radiator cap pressure rating???
Hi Bart,

Thanks for posting that information! I have never seen the explanation of the purpose of the radiator cap explained before. That might explain why some Tbirds overheat more easily than other ones during slow moving parades. I have not run Yellow Rose in a parade yet. I just checked my radiator cap to see what it is. I thought it was a 13, but I see it is a 14. Hopefully, with that cap and the metal shroud around the radiator now being in place, I will not have that problem. Sooner or later, I will probably find out. Maybe those who have had the problem, if not using a 15psi cap, should try one...:)

KULTULZ 01-27-2009 02:07 PM


Originally Posted by bcomo (Post 28135)

I wanted to add something about the purpose of a radiator cap.

The purpose of a radiator cap is to raise the boiling point of the water/antifreeze mix.

This is 100% correct. If one is concerned that the correct pressure cap will blow out the cooling system, it is time to perform a little service.

Additionally, the purpose of 7# caps was in the days of using either water or an alcohol based coolant. On this time period systems, it will not only cause spitting but will ingest air on the intake, creating air pockets and creating even more problems.

The AC radiator cap being correct for this period is interesting. I always cringed from anything not marked FOMOCO.

tbirds8 01-27-2009 06:42 PM

I know the 7lb isn't going to work in the summer. It will start spitin up. but I bought this car on flebay with a stuck motor and everybody gave up on it. From a ford dealer no less. Has 80000 on it and runs FINE. Been sitting since 1977. Took a little playin with it but you wouldn't believe how this car runs and everything works. (no clock). It's a 66. I think my 60 will run it down. But then I have figure out how to stop.

Patreer 06-04-2018 11:02 AM

Good day from Montreal,

I had to change my heater core recently and since I noticed the coolant tempature is higher then before (I have a gauge, a 180F thermostat and a 13# radiator cap).
Temperature used to be at 180F point all the time, but now I see that thermostat opens when engine warms up and after running for 20 minutes or so temperature climbs in the 195-210F range.
Yesterday we had 75F oustide and my coolant temp was more than 200F.

Even on the highway it does the same as if themostat was not operating.

I'm worried about overheating.
Should I change my radiator cap to a 15#?

Forgot to mention I switched to Shell Rotella T4, 15W40, if it may have an impact.
(1960 Tbird, 352)

Thanks for your help

jopizz 06-04-2018 11:26 AM

Changing the heater core should not cause your temps to be higher than before. My thought is that you have air in your system. Make sure you run the car with the radiator cap off and the heater control on hot to bleed any air out. I would not go to a 15 lb cap. The higher pressure will only put stress on your engine and radiator. Stay with 13 lbs. I've used the same oil many times and I haven't noticed higher temperatures.


Patreer 06-04-2018 11:42 AM

OK Thanks John
I'll do this and see if it helps.


simplyconnected 06-04-2018 01:31 PM

Did you ever do any canning? Canning produces a vacuum all by itself. Your radiator uses the same technique; first coolant is heated but when it cools down, the air inside contracts into a vacuum (provided your radiator cap has a good seal).

Cars with radiators LOWER than the engine (like Corvette and Escort) have used this technique for decades. It's important for those cars to have coolant in the aluminum heads at all times or they crack.

Over a few heat/cool cycles, your heater core should displace the air bubble with coolant. This is evidenced by finding a lower coolant level when the system is cool enough to check. Modern cars use the overflow to pull liquid back into the system. That's why their systems are always 'to the top'. - Dave

Patreer 06-04-2018 01:51 PM

I just did the test and clearly heard the thermostat pop while opening. I'm now waiting for the coolant to cool down. leaving the cap open.

Patreer 06-11-2018 11:35 AM

OK, I have been testing the bird several time now and it's still runiing at 210. Should I worry?

Patreer 06-22-2018 02:45 PM

Hi All,
Please let me know if ok.

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