Will a 6 gallon tank from a 2007+ Road King fit on a 2004 Road King?

 

It should.
2008 had the 6 gallon tank on the older frame.

 

What year did the frame change to make the tank swap impractical?

 

As far as I know 2008 and newer have the same tank.
2009 and newer have a different frame.
I am unaware of any differences between 2008 and 2009 for gas tanks, maybe someone else has something to add.

I have been wrong before.

 

I'm not an expert and don't take this as 100% FACT, but this is what I know... 2007 and older touring models are all definitely 5 gallon and 2008+ are all 6 gallon.
I've been told that the changes made to the 2008 frame included a longer top bar, (and also the different seat tab configuration). This was to facilitate the larger tank, which BTW, does not have the cross-over hose at the front like the earlier models.
Having no cross-over hose makes me think the pickup inside the tank is also different, which makes me question if the fuel pump assembly and the electrical connectors are the same (I know the electrical can be easily modified, but it's another line item on the list to address).
I'm concerned about the placement of the mounting tabs being very different, and I suspect there would be no easy/direct way to add one without modifying the earlier frame mounting points to fit the bigger tank.

If you figure it out, I'd love to see how you accomplish it.

I know that there was an aftermarket option for a larger tank, but I believe they are NLA, thus very expensive and difficult to find used.

 

https://www.hdforums.com/forum/touri...ml#post5711106