I recently bought a shower filter from amazon which claims to have 1/2" thread as per North American/ European standard (top in the image below). However, despite being the same radius as my shower's thread (bottom in the image, it is a Mildon All Directional Fixed Shower Chrome 3000AS if that is relevant), it does not screw in at all. I've attached an image below that shows both the thread types in their male form. I initially thought that the problem may be that the thread of the shower filter I purchased was BSP, and my home thread was NPT, since I went to the store to try on 15 mm threads, and they all worked with the shower filter, but none fit on my current shower head. I also brought out an old shower head I had and it screws on perfectly with the filter, but is unable to screw onto my current shower. Since I am in Australia and BSP is the standard, this would make sense. But the filter being in imperial units and North American/ European standard, would seem to suggest NPT? Or perhaps this is not the issue at all? I went into a bunnings but didn't receive much help.
