Sorry people, but whenever I install an antenna I just throw them away. They are a pain and serve no real purpose. The mount is sealed by the O ring underneath. A quarter wave whip will seal the top of the mount by its rubber "cone". Gain antennas will have an internal gasket or O ring in the base. Installed correctly with a little silicon grease, they will be leak free (until that tree limb at 50mph hits you).
Antenna kits ordered from Motorola have never contained this outer gasket. The only thing I see they are good for is to keep you from cranking the chrome nut on a 1/4 wave down way too tight. $8 for a replacement? Rather spend my money getting my mic clip powder coated.