Watch the video for an overview of the adapter.  The adapter is 2" nominal, but measures approximately 2.375" in diameter.  There are 11.5 threads per inch by the standard, but when measured, it appears as if there are 10 threads per inch.

 

The adapter will not fit Schutz brand valves.  Check your valve to see if says "Schutz".  The name is usually molded into the handle.  The Schutz brand uses a metric thread that is very similar to the US size.