Help with adaptor for old 1960s ball end shower arm
My parents bought an old house from the late 1950s, complete with a shower arm with a ball end. My mom would very much like to have a handheld shower head, but of course no one makes shower heads for our type of shower arm anymore. We would love to just change out the shower arm to the standard 1/2" one, but my dad is afraid we'll break a pipe in the wall (which we just can't afford right now).
I went out and purchased an adapter for our 1950s American Standard shower arm, which changes it from a ball end to the standard 1/2". When I screw the adapter + the new shower head into the captured nut, it only screws maybe halfway down the adapter's threads. This results in an extremely loose-fitting captured nut (when the original shower head is screwed in, the captured nut fits snugly against the ball end), and water squirts everywhere from above the captured nut. The only way to get a decently tight fit between the adapter and the captured nut is to stack about 4 or 5 o rings between the adapter and the ball end. My problem with this method was that if the o rings twist or shift, water comes shooting out again.
I do get really good pressure in the new shower head (even with the extra spray of water from the shower arm), but I'm afraid the water will leak back into the wall and cause rotting.
Does anyone have any ideas of how to help the adapter get a good seal with the shower arm?
I hope the pictures below aren't too big.
The adapter (and where the o rings sit):
The old shower head:
The adapter screwed in (only 1 o ring inside):
The old shower head screwed in: