Lining Up Threads…..

[Unregistered-Dodgeman]

If the origonal plumber did not fasten the piping in the wall rigid to a stud or some other bracing, it is possible the elbow inside the wall moved a little when you unscrewed the old arm. This will make it difficult to line up the threads.
Try a couple of things. Make the hole where the arm goes into slightly bigger to give you more room to line up the threads. Here I am assuming the arm comes out through drywall, not tile. If this is a tiled wall, this will be difficult to do. Another thing is to take a coat hanger and try to hook the piping in the wall to push or pull it back into position while you are threading the arm on.
The last resort is to open up the wall on the opposite side of the shower by cutting out a piece of drywall the width of the studs and 8 to 12 inches high. This will give you access to the plumbing. Of course, then you will have to replace the drywall, tape the joint, and use joint compound to finish everything. It is not as bad of a job as it sounds, and if the access is in a closet, the repair doesn’t have to be “perfect”
Good Luck!
Dodgeman