Shower arm width gotten bigger over the last 30 years?

Ive noticed after the last couple of months that my shower head has not been holding on to the shower arm no matter how much teflon tape i put on there. I asked my FIL to look at the threads thinking they might be stripped on the shower arm and sure enough they were. So we took out the original shower arm and i went to Home Depot to buy another one...Came home and went to put on the new arm and the width is wider than the original and wont screw in which of course now I cant replace the new one....What do i do??? I need a shower..lol

Even money that you BROKE OFF the old threads inside the elbow in the wall. Digging them out is a tedious but very doable job.

There has only been ONE "width" for the last century at least. You have different problem and a broken thread is the most likely one.

The shower arm just twisted right off..The new one wont even fit in the hole...How do i clean it out?