more gaugery

In my palace of earthy delights i.e my barn,wherein live all the landys and lorres in bits I have found a nice smiths cappilery oil pressure gauge.The union measusures around 16mm across the threads,dont know the pitch.However the stock oil pressure switch measures circa 9mm across threads.Obviously I need a tee piece,but what is the thread on the stock unit?The Wiring Prods Tee is 1/8" NPTF and they list a bunch of adaptor kits....any advice/experience here?

thanks from the pikey yokel

shocksi

So no one has fitted gauges to a 300?C'mon guys,a little sympathy for the mentally challenged here!