Does radiator cap need to be air tight?

[signsup]

Got nice new nickle radiator cap that I hate to paint OD green for my WWI ambulance project. Have older painted cap that I would prefer to use, but someone has drilled a very small hole right at the top. Either to let off steam, or to allow air into the system or to mount something. Can I use this cap as is? Presume I can JB Weld up the hole if needed. But the fact that it has a hole already, I'm thinking about a small eagle or statue of liberty flag topper on the cap for parades.
I'll hang up and listen.

[Mark Gregush]

More than likely someone drilled the hole because the overflow tube was blocked. If you are going to paint it, solder the hole closed or do as you suggested, add some sort of mascot to the cap.

[rwephill]

The Model T radiator is not air tight. It is not pressurized like a modern radiator. In the picture below you can see a tube coming out of bottom of the upper tank. It sticks up inside that tank to a height just below the filler cap. If things get hot and bubbly in there any pressure that builds or water that rises above that tube simply is redirected downward to the bottom of the radiator and out onto the ground. Better to have it exiting below the car than up higher where it sprays on the windshield, driver, passenger, etc. In your case, a hole on the cap would allow this to spew out upward and not exit the radiator as designed. Definitely seal the hole. It won't really hurt anything, but it will make a mess of your car.

Note that not all T radiators do not have the same exact tube routing. Mine goes nearly straight down, while the one shown bends to the left and goes down the side of the radiator. You can blow some air up through yours and see if it is clogged as Mark indicated.

[Charlie B in N.J.]

Nope. Not at all.

[DanTreace]

Yes, the cap has to be snug with the gasket as Ford supplied, but better results to keep coolant in the radiator is to use a modern plumbing rubber seal for $1.78 at ACE Hardware.

Plug that hole or add something, if left open coolant will spit out that hole. Be sure to seal the stem of what you add with o-ring or gasket otherwise you will have spits and spots on the windshield or spits and spots of hot coolant splashed on your face at road speed.

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