Review on Angle Simple Pressure Regulator Showerhead by Alex Prince

I bought two of these flow control valves 6 months ago. All fit perfectly with a standard Kohler shower head 1/2" NPT threads in two bathrooms. I repeated the order for two installs at a friend's. Both parts looked normal but once started the internal threads started. I measured the external thread and found it was 0.005" less is than the shower nipple; when viewed under a magnifying glass there were no obvious burrs on the threads. I sacrificed one by threading and screwing into a known nipple 1/2". and called for reinforcements. Note that this was with teflon tape on the threads. They were replaced two days later with the same result. While they look correct and well made, the only conclusion I can draw is that the male threads are not cut correctly for a standard 1/2 NPT fitting. I cannot recommend them due to the apparent quality issue that makes them unusable for so many customizations.