Going thru adapters/thread sizes on my 88 RRC w/ 3.5 EFI v8 now too. My oil pressure sender is 1/2"-20. My temp gauge sender is 5/8"-18. Seems the diesels have different sizes through the model run.
I could not find this info accurately online. As far as I can tell, what is on mine (US market RRC) is stock, from the factory, hasn't been resized or component swapped during it's lifetime.