Help with Shower Arm Thread

[Jds43232]

I'm sure this has been asked a million times, but I can't find the answers. There is so much to go through.
So I'm trying to put a better shower head on in my cousin's shower. The one I gave her fits mine just fine. It is the standard size. Hers is a 20 year old double wide and I do not want to replace the entire shower arm.

The trouble is that it just doesn't QUITE fit. I googled this issue and I've seen where people say there is a slightly smaller shower arm out there that standard shower heads don't fit. Well, what can I do? I tried extra Teflon tape and it's just not quite enough.
Is there a fix? Is there an adapter I can go ask for? I don't know what this weird off-size is called but she wants me to just forget it. Her shower head is so terrible I don't want to forget it.

 

[havasu]

Your best bet is to take her shower head off and match it exactly. I'm also sure if you knew the exact sizes you needed, you could locate an adapter on the Internet, but you need to know the size exactly.

 

[Mr_David]

Buy a cheap caliper. $10

Measure the outside diameter of both shower arms.

A standard arm is 1/2" IPS.
I have one in my hand and it measures 13/16 OD.

 

[Jds43232]

I can try that, thank you. Yes the one I'm trying to put on is a standard 1/2".
Hers is just the slightest tiniest bit too small for the threads to catch but teflon tape isn't making it catch either. So yea I can try that.

 

[havasu]

Remember, mobile homes can be a completely different beast.

 

[Jds43232]

Yes it's frustrating. She lives far away and I'm not going to be back there for about 6 more days and I want to take care of it that day.
It's one of 2 things. Either the threads aren't deep enough on hers to catch a standard 1/2" or it's just the absolute slightest bit smaller. I think it's the second one because the threads look ok to me.

She has the original shower head in her shower and it's SO BAD. Cleaning it doesn't even help...but it takes her almost 10 minutes to rinse shampoo out of her hair. Just ridiculous.