I also recently found out from North Idaho Wolfman that it is 1/8" BSPT.
If you are by chance looking for a test guage for it then you can buy this one http://www.harborfreight.com/engine-oil-pressure-test-kit-98949.html
at a reasonable price and it comes with several adapters including the one you need. That is of course should you be needing a kit.
Good luck.