rad. pressue caps.

dick lyons

dick lyons

Senior Member
hey guys. ive been reading old threads for weeks now and i just seen one o rad caps. went out to the garage and looked at mine,13#!!! wow. guess thats WAY to much. 53 merc, with smaller hoses than 59ab, a 53 merc rad wouldent fit in a 48 ford,would it? waht pressure should i be useing? thanks, dick from del. b.t.w., no leaks to be seen anywhere.
 
4 lbs is good but the BIG PROBLEM is MOST caps don't seat down on the brass seal washer down at the bottom of the neck. If you line up the cap locks at the top when you push the cap down you can fell the spring tension of the rubber end of the cap hitting the brass seat down in the neck before the upper part of the cap hits. If it don't hit and seat you don't have a pressure cap. I have had to put a red rubber washer down on the seat so the cap seals on it on almost all the old Fords I have. G.M.
 
old henrey & g.m., the cap does seal on the neck. if i have the smaller hoses, that would mean the rad oulets are not stock 48, right? but regardless of that, 13# is way too much then. i have driven the car about 100 miles since i got it.been tacking care of other issue and repairs. what would the consequences of useing 13# cap? it never has run hot. iwill get another cap. no more than 4#??. thank you for taking the time to reply. we all value youe knowledge. dick.
 
dick lyons said:
what would the consequences of useing 13# cap?
Click to expand...

On my Pike's Peak trip I put on a 10# cap to keep the water in at high altitudes. The consequence was that water started squirting out of the hoses past the hose clamps. The system is just not made to hold that much pressure.

Increasing the pressure of the water raises its boiling point such that the engine will get hotter before boiling the water out. Here's how it works:

In your state all elevations are close to sea level. At sea level of c. 15 psi water boils at 212 deg. So, if you had no radiator cap the water would boil out at that temperature. Adding a 4# radiator cap raises your boiling temperature to 225 deg. and a 13# cap raises it to 245 deg. So, as you can see, the higher the pressure of the water the hotter it can get before boiling out. That's about it other than where the old system designed for 19 psi (sea level psi of 15 plus 4# cap) is going to blow with 27 psi in it.

You may have noticed that modern engine temperature gauges don't redline until 260 deg. That's because modern engines have a 15# cap that keeps water in to 250 deg.
 
19FORDY 7 OLD HENRY. THANK YOU. I WILL GET A NEW 4lb.cap. napa is just 5 min. away,lucky me! there are still some things im not happy with, but with guys like you willing to help. the guys less schooled on the wonderful and misterous flathead V8, i will get there. thanks again, dick from del.
 
You can run 100 lb cap and not have a problem until you have an over heating problem. The hotter it gets the higher the pressure in the system gets. First the radator tank will pucker up and then something will break and steam and hot water will shoot out. This can burn or harm the person it comes in contact with. A 4 lb cap is all that is required on these engines. A 4 lb cap will hold pressure produced by about 212 degrees. The pressure cap seals the over flow tube and allows the coolant to be filled up to the filler neck provideing better cooling and stops the lose of water. The cap is also a safety valve that opens at 4 lb pressure releaveing the pressure in the system. Pressure can also be interduced in the system by a compression leak. Again I caution that most pressure caps don't seat at the bottom. G.M.
 
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