Doesn't change the fact that if you have a 216 and you decide to do a test run on an engine stand, and you go to plug the water temp hole, (or you want to put an electric sensor in the hole) you have a problem because everything else on the engine is NPT and that one is UNF. Maybe what you could do is just stuff a cork in there? LOL! Or.. just wrap a 3/8" NPT plug about 50 times with teflon tape. Aftermarket came to GM's rescue on this one. Nothing a lathe and some special taps wouldn't resolve I suppose as the supply runs out.
