0# Radiator Cap?

I ordered a new cap for my 51 GMC with the stock radiator. It's a 0#. My motor is a 58 Chevy 261. The truck is running hot - the rad is boiling and overflowing. I had the rad cleaned out before I installed it and the water pump is new. t-stat i new too. Could I need a different cap?

 

I would think you would need at least an 8# cap. The system does need to pressurize to cool properly.

 

For every pound of cap pressure it takes water 3 more degrees to boil. So with an 8 pound cap your radiator would not boil over until the temperature reached 236 degrees instead of 212 with a 0# cap.

 

Thanks fellas. I pulled out the t-stat and checked it in hot water in the kitchen and it worked so I put it back in. I put a 7# cap on it and it's staying a constant temperature now. It's running about 190 to 200 according to my gauge.