What psi should my radiator cap be rated for???

[Bartenfeld]

What radiator cap psi rating is stock for a 66 with a 283?? The 283 was removed, but thats what the truck originally came with. It still has the stock radiator, but the engine has been replaced with a 305. For what reason would you want to run any different psi rating??? They have 7, 13, 16, 18, and 20psi at the local parts store......

[Alan's Classic]

Moved ya to engine and drivetrain, someone will chime in.

[storm9c1]

13 or 16 psi is usually what I run on all of my vehicles from that era. It's usually the stock value for the brass radiators and a stock system (not sure what year the 7s were used up until). I wouldn't go any higher than 16 unless you have an upgraded cooling system.

I don't know if anyone else discovered this, but if you use the standard Stant cap (and NAPA brand which are rebadged Stants), be careful. I haven't found a 16 psi in two years that actually holds 16 psi. Many failed to build up pressure beyond 5 psi (same application where the 13s worked flawlessly). I found many 16s are made with one less spring now (to save money???) and just don't work right. I've taken a few back and explained this to the parts store guys, who just look at me baffled. The 13s still have the extra spring and they work right. You can physically see the difference when you look at a good and bad 16 side by side (or even compare a 13 to a 16), the missing spring causes the bottom check valve to dangle aimlessly without being pulled up against the rubber seat. Only on the 16s. The ugly (but functional) Lev-R-Vents seem to work fine too.

[fleetsidelarry]

IIRC (and my brain may be way off on this) each # of pressure raises the boiling point of water something like 3° F, so a 13# cap would keep your system from boiling over 'til it reached 251°. I wouldn't worry about running any thing more than stock. I think I run a 13 on mine. a 7# still gives you 233°. keeps you from boiling over, doesn't cure overheating

storm makes an excellent point about caps holding pressure, they can look fine and not work worth a hoot, as I found out recently when a friend suggested I check the one on my street rod.

[fleetsidelarry]

And to answer the original question, the '63 service manual says 13# for pickups and the '66 supplement says no change to cooling systems from '63

[Bartenfeld]

Ok, I put a 7# cap on it the other day just to be safe. I guess i'll just keep that one. Any objections? Going off of fleetsidelarry's post, I shouldn't really be getting up to 233 anyways.

[storm9c1]

Originally Posted by Bartenfeld

Ok, I put a 7# cap on it the other day just to be safe. I guess i'll just keep that one. Any objections? Going off of fleetsidelarry's post, I shouldn't really be getting up to 233 anyways.

If a recovery tank was never added, then be sure to check your coolant level often. That 7# is quite low and the cap may release often enough to vent a little steam more often than you think, especially if you put it to work.

[Bartenfeld]

yeah the 7# cap has been releasing a little bit here and there. I think I am gonna step up to the 13# cap.

[Warrens69GMC]

Just FYI, here in AZ I run a 180* thermostat year round, and run a 13lb cap in the summer (runs at 180*) and in winter I run a 16lb cap (runs around 195-200) for more winter heat.