Slingshot fuel gauge thread size

[Buddy]

Hi lads

just wondering if anyone knows the size of thread in the tank that takes the fuel gauge? I have a turbo slingshot but originally the water drain in the tank has been cut as used as fuel return, but had an issue lately with water getting in, have always wanted to Change it, so thinking about another tank and make up a fitting so I can screw in a return pipe where fuel gauge would go (isn’t used) 

 

thanks