Springer fork center bolt size?

[GrantYoungPhoto]

I have lost the nut on my springer and need to replace it. It doesn't have to be the acorn but necessarily, just need to get it back secure.

 

[B607]

If it's a Schwinn, it takes a special size nut. Proprietary threads. Typical Schwinn. Gary

 

[kingfish254]

Can you narrow it down for us. There are about 30 different springer forks you might be talking about.
Pictures would help.