There is such a thing as a "combination thread" thru-hull. A combination thread is a half arsed way of making a ball valve thread onto a standard straight threaded thru-hull. Essentially the tips or peaks of the threads are machined off so the ball valve can thread over them and get more thread purchase.


While this is clearly better than using a regular straight threaded thru-hull with a ball valve it is still a "shortcut". One major draw back is that you can not shorten the thru-hull to minimize interior height. The longer the stem inside the hull the more leverage there will be to potentially snap it off. If you MUST use a thru-hull with a ball valve threaded on please source some combination threaded thru-hull fittings.


If you click on the picture to make it larger you'll clearly see how the first few threads are shaved down. This does NOT make it an NPT thread just a band aid approach to get some more thread contact area. This design was likely driven by large production boat builders in an attempt to save a few pennies. While certainly better than mixing NPS to NPT I would think twice before using a combination threaded thru-hull with a ball valve.