KB2 with T9 4 speed will not shift into reverse

I have a 1948 KB2 with a Borg Warner 4 Speed. It does not want to seem to shift into reverse. I have had the top cover off 20 or more times. The shifter will shift into all gears including reverse with it not mounted on the trans. When I mount it on the trans it will shift into gears 1 thru 4 but not reverse. I seem to be able to slide the reverse gear manually into gear with the cover off and the other two main gears in neutral. Some have said the reverse fork is not in the slot but I have confirmed that it is. What am I missing?

Since, I believe the inter-lock is in the cover, the only thing I can think of is one of the other gears (shift rails) is locking out the reverse rail, when on the the trans itself. "bent shift fork'? something out of position? Shift rails in wrong position (IE 3-4 in place of 1-2). Not much of a help, but my eyes aren't on it. If you can move into all gears with the cover off, and can't with it on, that points to the cover being the problem.

I am starting to think the same thing as it seems like a hard stop when trying to slide the shifter all the way to the right. I seem to have the 1-2 and 3-4 gears in the neutral positions and the forks seem to line up fine I am wondering of one of them is getting rotated slightly when mounting it. I have never seen one this touch if that is what it is. I had also read on one forum to put the shift cover on the trans while the trans and shift cover are in 2nd gear? I fiddled with that but did not seem to help. Have you ever heard of that?

I found the issue but not necessarily the solution. The shifter forks for 1-2 and 3-4 rotate and when they rotate one direction a little it locks reverse out. I tried to force those forks to the side that does not cause the reverse lockout but no matter what I do when the cover is put on the transmission it locks reverse out. I assume there is something not right with the springs, balls and pins in the shift rails that is causing this as it has not been apart. I have another trans so I am going to try and swap the shifter from that trans over to this one. If anyone knows specifically what would cause this please let me know.

Is it possible the shifter forks are not located properly on the shift rails? From what I know of most covers, the lock-out's are just "holes" in the rail, they do not locate the rails. How the fork is on the rail should hold the rail in the correct position as the rail and the fork are bound and move together. For the rail to want to "turn" the fork must move with it, so finding why the fork is not sitting in the correct place is where I would start. There is most often a set screw holding the fork to the rail.

Thanks for the reply. The set screws are wired and have not been touched or moved in many years. They still have wires in them as a locking mechanism. The rails were stuck initially from sitting and the moisture but I just oiled them and worked them back and forth and off the trans they seem to move and shift fine now. I agree that it could be an issue of where the gears themselves are not in the correct position when the forks are inserted though. I have the 1-2 and 3-4 gears in a position where neither are engaged and the spacing appears to be correct for the forks with the shifter rails all in neutral. I have a KB service manual and it has a trans section showing the T9 but no specific info on how to align it to assemble.