Help with an outside faucet

The backflow preventer on my outside faucet has started leaking. I took the piece to Home Depot and a wholesale plumbing supply store and neither could replace it. The male end to the hose is regular size with regular threads. However, the fremale end to the faucet has very fine threads inside. It is also slightly larger than 3/4", but not sure of the exact size. The piece has no markings and the faucet is simply stamped with USA. Any thoughts on how to replace this or or find a piece to convert the pipe to 3/4"? Thanks for the help.

valve

That backflow preventer is part of the faucet and was made strictly for it. Unless you can find a faucet that is exactly like it, from the same manufacturer you will have to replace the faucet.

Hard to say without knowing the brand.... To take a guess, If your faucet has a green oval handle then it possibly could be an Arrowhead Brass faucet "ABP" and the replacement part for that paticular faucet would be a PK1390. If that is the faucet you have and no local suppliers have the part, it is available online.