fuel tank sending unit gasket

[bigmelton]

I have an 1984 bronco 2. When I fill it up it leaks out of the cap on top where the sending unit is. The o-ring is to big. What size is the correct one or what will work to seal it.

 

[dirtcowboy]

http://www.lmctruck.com/icatalog/fr/full.aspx?Page=102

Numbers 2 and 3 84 BII.