Radiator Cap & overflow system explained

[steveg]

Several years ago I added a rubber gasket inside the stock 7lb radiator cap and an MG overflow tank. Have noticed the system will keep the Healey radiator tank almost full to the brim. When the system cools, coolant is pulled back from the tank into the radiator. Keeping the overflow tank half full adds to the capacity of the system.

Knew this worked but not why. This video discusses the small return valve and how it works:
https://youtu.be/0JdDWWoX-70

Return valve on stock 7lb cap.

Hose input to right of cap goes to brass pipe to bottom of tank.
I leave 2nd 7 lb cap on loosely. According to video above it could be tight as return valve would equalize to ambient pressure.

 

[Lotuswins]

Hi Steve,

Thanks for posting the video. It does increase the volume of the coolant, but I don't think that is the major advantage. In heat transfer, air entrainment is your enemy, and the closed system described pulls the air bubbles out of the system, enhancing the heat transfer. So, as those bubbles break away from other areas of the coolant system over time, they are burped out through the expansion tank....neat!!