Sometimes they are 2" standard pipe thread - if you are lucky.
They may also be 2" acme pipe thread - corser thread but easier to hand tighten.
There may be a clamp on the valve which you can take appart.
I've been lucky with a couple and was able to reduce down with the pipe thread bushings. I also have a 12 volt pump set up on one with 3/4" hose outlet - this is good for the tractor too, I can piuck it up with the forks and move to where I want.
Good luck. Grant