Radiator cap question

[70chevytruck]

Replace radiator and water pump and hoses this weekend.....made it a closed system by adding a overflow jug......16 lb cap on it now....suggestions on new cap? Should I go to a lower 13 lb cap?

Know how everyone loves pics.....

[70chevytruck]

More pics...

[cdowns]

If you leave the 16lb cap the high pressure helps blowup the radiator and heater core in far less time// that's what i was tought about 50years ago and still live by that rule

[72MARIO]

What do you mean by blow up ? Spring a leak ??

[Stocker]

That is a beautiful truck, inside, outside, and under the hood!

My memory thinks my '72 calls for a 15# cap.... might wanna verify that. I also think I went with a 16# replacement. Not sure why (it has been a long time) but maybe b/c that's what my local NAPA store had. And I don't for a minute think a one pound increase in maximum pressure will cause an otherwise healthy cooling system to destroy itself.

I'll see if I can look up anything and then post on this thread. Did I mention how nice your truck looks?


Edit: Yup, it has a 16# cap. And I'll guarantee you I didn't just pull that number out of my hat. Still need to verify what OEM was....

Edit again: just checked three different online sources. All three call for a 16# radiator cap. I think you're good to go!!

BTW I have owned my truck for 40 years. The OEM radiator rotted out and was replaced several years ago, and the heater core was replaced a number of years prior. Neither had blown out or split apart, just rotted with age.

[Madpiper7777]

Beautiful truck!!!
I just bought my 72 longbed a week ago. I'm new to older vehicles (or any vehicle other than oil changes) but I came across this thread because the PO had installed a 7 psi radiator cap and needed to check the forum if there would be any other reason for low psi cap.
I'm literally in the middle of a radiator flush and thought i should check in to see if I should go with the 15psi cap (handbook recommended btw)
Looks like I'll have to pick 15psi ASAP
Im guessing that'll cut back on the coolant puddles in the garage as of late 😜

['68OrangeSunshine]

I am not a heating and cooling expert, but I believe the stock options were 13Lbs for Standard trucks, and 16 Lbs for Trucks with Air Conditioning.

[Mac's]

I believe the higher the pressure cap, the higher the boiling point of the coolant. A weak radiator, hose or heater core is more likely to leak with higher pressures. If you are not running hot I would run a lower pressure cap and not stress a 40+ year old cooling system. If you have an updated system run the 15psi. I am sure if I am incorrect, somebody will correct me. Not trying to be funny; I learn something every time I read theses forums and often find what I thought was right is not.

[Freddo]

I'm no expert on these cooling systems by any means, but I would chime in with Mac's & CDowns on this one at least if you are running an old radiator. My dad owned a radiator shop for many years & it was common for him to suggest a lower pressure cap to people who came in with an older car with a tired radiator. These weren't the people taking them to the drag strip or guys pulling big trailers. These were people with their 12 year old daily drivers, with a radiator that had seen better days They couldn't afford a new one, but could afford to boil out & patch up the old one & keep it alive for a few more years. Some of the 'honeycomb' (as he called them) radiators that came on some of the 50's model trucks would have 4 lb caps and if I remember correctly, it was common for him to put 7 lb caps on 60's & early 70's model trucks with old cooling systems. If you've put an entirely new cooling system it would make great sense to follow the factory recommendations, but if you are still using a 40+ year old radiator or heater core, it might put less stress on the entire system to use a lower pressure cap.

[Madpiper7777]

Thanks for the quick reply guys!! Honestly I'm not sure if the cooling system is old or not. I have no over heating issues at all. I'm just doing a full preventive service to see how she is and to get better aquanted with her. Maybe the better solution would be to install a proper overflow tank and keep an eye on it before I go changing everything. If it ain't broke, don't fix it right? Thanks guys!!

[piecesparts]

The discussion about the boiling point of the fluid in the radiator changing with a different pressure radiator cap is true. The higher the pressure rating, the more the engine will support without boiling the water out into the bottle or onto the ground. Now let's talk about the truck overall: If you have a good engine, truck, and drive it a lot; then upgrade the components that support that truck. Look at a new radiator and heater core. The more I get around the country, the more I see people cheap out on their builds and the more I see them sitting on the side of the road with a really nice machine steaming and puking it's guts out on the ground. Spend a lot of money on the truck and skimp on the cooling system---NOT COOL, in my eyes. Make sure that radiator cap is designed to be connected to a catch bottle. That way the fluid moves both ways.

[geezer#99]

Originally Posted by 70chevytruck

Replace radiator and water pump and hoses this weekend.....made it a closed system by adding a overflow jug......16 lb cap on it now....suggestions on new cap? Should I go to a lower 13 lb cap?

Know how everyone loves pics.....

Attachment 1420377

Attachment 1420378

Overflow jug or a closed return system?
There is a difference.
Where's the vent/overflow line on your overflow jug?

[EARNHARDT#3]

That's not a closed cooling system. A closed cooling system has a pressurized coolant reservoir. A closed system circulates coolant through the reservoir. Yours simply fills the reservoir as the coolant gets hot and expands. Then as it cools it siphoned the coolant back in.