Motor meter winged radiator cap

[1930Ford]

Cannot close the hinged cap with any type of rubber ring seal/gasket. It closes without the seal/gasket. Tried loosening the top without any change. Previous owner used what appeared to be a cardboard gasket. A regular cap works fine. Any thoughts?

 

[nkaminar]

On my car I use an oversize O ring. I never have to fill my radiator so I use a little silicone sealant. You may try to form your own gasket using the silicone sealant. A light film of oil will prevent the silicone from sticking to the cap and you can clean the radiator surface to have you cast gasket stick there.