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  #1  
Old 01-23-2009, 03:07 PM
trim code 76 trim code 76 is offline
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Question 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.
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Old 01-23-2009, 03:43 PM
bcomo bcomo is offline
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I have a 13 PSI cap on my 60.

I can't find it in the 60 shop manual either.
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Old 01-23-2009, 03:46 PM
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Default 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.
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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
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Last edited by YellowRose : 01-23-2009 at 03:49 PM. Reason: Additional comments
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Old 01-23-2009, 05:34 PM
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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.

john
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Old 01-25-2009, 12:59 AM
FeFranco FeFranco is offline
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Quote:
Originally Posted by JohnG View Post
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.

john
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.
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Old 01-25-2009, 03:38 AM
trim code 76 trim code 76 is offline
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Thanks guys!! Knew I would get the right answer from you.
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Old 01-26-2009, 08:22 PM
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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.
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Old 01-27-2009, 01:26 AM
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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.
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Old 01-27-2009, 10:50 AM
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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.
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Last edited by bcomo : 01-27-2009 at 11:10 AM.
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Old 01-27-2009, 12:45 PM
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Default 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...
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