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Posted By: Gmetzger Coolant Pressure - Mon Sep 13 2021 05:19 PM
Hi I recently acquired a 1951 3100 there was a radiator but wasn’t installed I brought the radiator to a radiator shop to get it cleaned and checked. The shoo stated everything was good but they weren’t to sure about the built in pressure relief valve. Does anyone know what the pressure is to build to. It’s a 235. The pressure relief is built in to the upper tank and has a copper tube soldered to the side of it
Posted By: buoymaker Re: Coolant Pressure - Mon Sep 13 2021 05:55 PM
The original 1951 Chevy truck radiator, made by Harrison, did not have a "built-in" pressure relief valve. There is a zero (see below, thanks Tim) pound radiator cap that twists on/off onto the neck on the top radiator tank. Soldered into the neck is a tube that runs down the side of the radiator that allows expanded coolant to escape (overflow) if the radiator is filled too full of coolant (coolant expands with heat). I believe it wasn't until 1954 that a 4 pound pressure cap was used. The old radiators can't stand a lot of pressure so I wouldn't go higher than 4 pounds.

If you do not have an original Harrison radiator, see if you can find identifying numbers or manufacturer. A picture would be nice.
Posted By: tclederman Re: Coolant Pressure - Mon Sep 13 2021 06:04 PM
in 1951, a 4 lb pressure radiator cap was used (3 1/2 psi to 4 1/2 psi) [chevy.oldcarmanualproject.com]

same pressure in 1954 [chevy.oldcarmanualproject.com]
Posted By: 68ironhead Re: Coolant Pressure - Mon Sep 13 2021 08:51 PM
Sounds like you have a GMC radiator with the built in pressure relief in the upper tank.
Posted By: Hotrod Lincoln Re: Coolant Pressure - Mon Sep 13 2021 11:50 PM
All it takes to check that relief valve is an air hose and a pressure regulator. Should you really be dealing with a "radiator" shop where they don't know how to do that?
Jerry
Posted By: Gmetzger Re: Coolant Pressure - Tue Sep 14 2021 01:48 PM
Thanks for the info the shop stated that the pressure relief was working just that it was active at very low pressure since it is only set at 4lbs that would be normal.
Posted By: Hotrod Lincoln Re: Coolant Pressure - Tue Sep 14 2021 05:30 PM
Since every pound of pressure on a cooling system raises the boiling point 3 degrees F., a #4 pressure relief will raise the boiling point at sea level to 228 degrees F. Most stovebolters would get their lacy drawers in a wad and have fainting spells if their engines ever got that hot!
Jerry
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