Radiator cap pressure

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Radiator cap pressure

Post by Ted FM 13 on Fri Feb 17, 2017 1:36 am

What is the correct pressure for the radiator cap? I just replaced my old 13# cap with a new one but it still loses a significant amount of coolant even with temps pegged at 180F. I have just been filling the system every time I drive it, happy that I'm not overheating. But I know this isn't right. So maybe I need a higher pressure cap? 15 or even 18#? what do you guys have?
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Ted FM 13

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Re: Radiator cap pressure

Post by Ted FM 13 on Mon Feb 20, 2017 3:01 am

Pressure rating of the cap is not my problem if, as I suspect, the cap is not the right size; it appears to be about 1/8" too short to actually seal at the bottom of the neck. I put in an O ring 1/8" thick but that didn't work. I put in a gasket 1/4" thick and drove it at operating temps and absolutely nothing came out.

So I'm wondering:

1) how can I find the right size radiator cap? I don't know what car the radiator came from and CARQUEST, Auto Zone and O'Reilly's can't find a cap a bit longer.

2) with the 1/4" gasket, have I jammed the cap to the point that it can't move to act as a vent? I think the vent movement is designed to be only 1/8". If there is some venting movement still available, shouldn't I have seen some coolant vented (after filling it and driving it at 180F)? I burped the system by cracking the thermostat bleed screw the highest point on the cooling system.

3) IOW, to help answer #2, at what water temp does venting start? I'm assuming it has to vent somewhat since the coolant must expand as it heats up to normal 180-200F temps from the 60F temps at filling.
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Re: Radiator cap pressure

Post by Ted FM 13 on Mon Feb 20, 2017 5:12 pm

A mechanic who's also a racer said the cap should not vent at normal operating temps and venting should start at 220F or some temp definitely hotter than normal. That sound right?

The builder sent me the parts list that Brunton sent; he said he used the 87 Hyundai radiator and he's been very meticulous with his records and memory. But the parts store had a much smaller cap for the 87 Hyundai, so I'm still at a loss about how to find a longer cap.
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