The Smiths
oil temp gauge on my car has a 3/8 BSP sleeve which draws the bulb down into a double male adapter, threaded 3/8 NPT on the other end.
I needed an adapter to attach my sender into a 1/2" NPT fitting when I moved it from the
oil pan to the remote filter line. The 3/8 BSP is about 5/8" OD, if I recall correctly and 19 TPI, so I couldn't even make one on my lathe with the change gears I have. Forget finding a threading die.
When I went looking for a 3/8 BSP to 1/2 NPT double male adapter, I found a website in New Zealand with a listing. I emailed them and they referred me to some tractor supply place in WA or OR, from whom I was able to order the part. I thought I had saved the info on the computer but can't find it now. If that sounds like what you need, shoot me a PM or an EM and I'll look around for the invoice.
Pegasus also has a few 3/8 BSP fittings; you could check their catalog to see whether they have what you need.
Lowell