mobeewan wrote:
The threaded brass fitting on the tank has a male 3/8 inch pipe thread. The outer diameter of 3/8 inch pipe is .675. According to your picture the pipe threads are just under 3/4 of an inch, so the threads have to be 3/8 in pipe threads. Half inch pipe is .540 in diameter and three quarter inch pipe is .840 in diameter. So your elbow you're looking for would need to be 3/8 in female pipe thread swivel nut x shark bite.
I'm teachable mobeewan, really, but how can the OD of a 1/2 in pipe be only 4-100's more than the ID, but the OD of the 3/8 in pipe be nearly double its ID? Heres a shot of the ID:

Heres something like what you're saying but the reviews are horrible.
http://www.sharkbite.com/product/swivel-elbows/