Hello, I have an MXB that has a female tank bung and an OEM female petcock.  Is the supposed to be some sort of coupler in there?  The petcock I had was English and had male threads, but I'm trying to go back to original.  
Thanks, all.

Here is a link to an original item on CMSNL. The nut shown also screws on to the outlet from the tank and is the effective link.

I'm assuming that I've got the right model year!

FUEL COCK ASSY for Yamaha - order at CMSNL

Yes, that petcock is the one I have.  It is female threaded, as is my OEM gas tank.  Had to go to a hardware store and manufacturer a coupler out of a brass fitting.  It's a mystery!  

It should thread straight on, maybe left hand thread.

My guess is the tank still had a coupling nut on it?  Or some sort of modification.  I can't remember ever seeing female threads on a stock Yamaha tank. Probably aluminum tank so sometimes the nut does get stripped/seized.

Often one side of the coupling nut is LH thread and one side is RH thread. 

My conclusion is that the male bung had stripped and someone modified it to female.  The PO had fitted it with a fuel valve that looked British and accepted the opposite connection. Thanks, all.  

Has anyone ever fit the MX 400 jug and intake/carb into the DT?  The air cleaner and autolube would need changes, but the MX is a beast, and I believe makes about 10 more HP.