I busted off a sprinkler head in my front yard and decided that, instead of paying like 25 cents at the hardware store for a replacement, I'd spend the next 5 hours modeling threads until my head hurt.
Fits 1/2 inch threaded pipe fitting. Thread pitch has been verified to work.
If you need shorter or longer, it's pretty simple to crop a chunk out of the middle and lengthen or shorten as needed if you have a program like 123d Design or the like. I included the 123dx file so you can tinker with it as needed.
The original had half as many ribs for strength. I figured that, with the fragility of PLA, I'd double the number to give it more girth. (giggle... girth...)
When printing, be sure to keep your support structures out of the threads or you'll have to spend the next hour picking tiny pieces out of them like I did.