Radiator cap pressure?

55_2_drive · Oct 20, 2021

I see there are a lot of old posts on this topic but nothing particularly fresh.

I'm running a 355 SBC that's fairly high performance. I have a brand new aluminum radiator and installed Southern Air - with a new heater core etc - in the car. So basically, all brand new parts.

I can't tell by looking at my current radiator cap what how many lbs it's rated at, and since I'm going to do some polishing of the radiator (strictly to enhance the cooling properties mind you) I want to buy a chrome cap.

My question is how many lbs should the cap I buy be rated at. I saw all kinds of answers from "stay with 6-7 lbs on an aluminum radiator" to "16lbs is fine"...so I'm looking for advice here.

mr55/56 · Oct 20, 2021

If you are using 65 year old cooling system a parts go 6-7 pounds. My 55 has a new heater core rated at 16 pounds by the manufacture and an aluminum radiator so I went 12. I believe the oem stuff when new could of handled more than 7 pounds but it now eligible for Medicare.

Had a few Tri 5's · Oct 20, 2021

Yes, the system will handle the 16 LB cap, except the heater core. Either swap it out for a new one or go with the 7 LB cap. Unless you have over heating issues, it will be ok.

carls 56 · Oct 20, 2021

7 to be safe.

sthec · Feb 3, 2023

Can a cap with higher pressure (like 16) force a small leak from the intake? Mine has started weeping up front between the intake and the head (pass side).

Pops @ Tri-5.com · Feb 3, 2023

sthec said:
Can a cap with higher pressure (like 16) force a small leak from the intake? Mine has started weeping up front between the intake and the head (pass side).

No...That should not be a problem...Many SBC operate with a 16 lb radiator pressure cap.

mr55/56 · Feb 3, 2023

Not unless the gasket is old, not properly installed or the wrong one. In other words it will leak regardless.

Fiddy5Coupe · Feb 3, 2023

Re-torque the intake bolts and if they have been removed put sealer on the threads.

sthec · Feb 3, 2023

So I re-torqued the bolts and changed the cap to a 7 lb and no leak. During my searches I also came across a thread about some of the old heater cores not liking that much pressure so I changed the cap too. I have zero cooling issues so don’t really see the need to have a 16lb cap on there anyway.

hotrodg726 · Feb 3, 2023

you said you have a souther air ac unit does it include the heaer core ?? if so a 16lb cap would be appropriate.

Rick_L · Feb 4, 2023

Leaking heater cores are over rated. Sure, an original 65+ year old heater core is at risk. It's at risk with any rating of the pressure cap.

New heater cores are at less risk than new radiators.

mr55/56 · Feb 4, 2023

Rick_L said:
Leaking heater cores are over rated. Sure, an original 65+ year old heater core is at risk. It's at risk with any rating of the pressure cap.

New heater cores are at less risk than new radiators.

I feel this way every time its brought up.

abdiver7777 · Feb 4, 2023

The boiling point of water at 7psi is around 232F.....

If you run a 180F or 190F thermostat and your system is operating correctly a 7psi cap should be adequate and less chance of leaks.

angs1957 · Feb 4, 2023

Leave the 7 Ib for now check how your cooling system is going since now you have no leaks.

Radiator cap pressure?

55_2_drive

mr55/56

Had a few Tri 5's

carls 56

sthec

Pops @ Tri-5.com

mr55/56

Fiddy5Coupe

sthec

hotrodg726

Rick_L

mr55/56

abdiver7777

angs1957

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