Problem with NPT threads on dual check valve?
Sounds like the threading machine was set a bit two large, many universal dies are adjustable. My Ridgid 535 I have to set the adjustment for just under the size marked on the gauge to get proper sized threads. You may be able to take the pipe to a plumbing supply store that can recut the threads. If a fixed jaw die is used the threads need to be cut the full length of the cutting die to get the proper taper. Usually the female threads are cut with a die if not deep enough the same problem can happen.