Folks I had to refer to a copy of Zeus Precision Tables, 1/2 x 13 is a Unified Coarse (UNC) thread, the equivalent Unified Fine thread is 1/2 x 20.  Or Machinery's Handbook but I don't have a copy yet, its on the way from eBay. The UNF/UNC threads were in very wide use in the automotive world - even in the UK.  UNF for nuts and bolts, UNC for threads in castings. Making screws to fit each other was a challenge until Whitworth created a standard table, I used to machine cut screws on my Myford lathe - but only imperial, I think most engineers just use what comes naturally these days if they grew up with imperial.. The interesting thing about the BSP thread is that the taper version is self sealing, whereas the metric equivalent isn't, which is why BSP is still in wide use today... Provided the pitch is correct thread engagement will work fine if there's enough clearance on the threads - so you can make a thread to size in OpenSCAD and scale it down [or up] in X and Y in the slicing stage. Making non-metric threads in OpenSCAD is just a conversion thing - so its not a problem doing 1/2 x 13 :-) so its all good. R.