I have a crack in the threads of the shower arm which is causing a leak when the shower is on. Does the shower arm screw on to the pipe coming out of the wall? I can't seem to find any instructions on line or in a book that shows how this can be replaced. I don't want to try and unscrew it and really mess something up. Any help would be appreciated.

That arm should be screwed into a 90 degree drop ear elbow which is attached to a stud, hopefully. You should be able to unscew the old arm, 3 wraps of teflon and screw the new one in place,

Do you mean the shower head? The shower head threads on to the shower arm. The shower arm is the pipe coming out of the wall. If the shower head is cracked at the threaded portion, you can simply unthread the shower head and install a new one onto the shower arm, using teflon as ball suggested. Simple fix. Please let us know how things work out.

Please describe the spot where the threads are cracked. It almost has to be the shower head if you can see the crack. Answer Lees question. Back to you, n Tom