mechanical water temp gauge

I put one in the side in one of the water jacket plugs, but it reads a little under the actual temp. I think water temp gauge probes are some special thread that's not standard NPT, but I would think an auto part store would probably have the right size. My gauge came with several different adapters that are standard NPT on the outside (I think the engine port is 3/8 NPT if I recall) and whatever the temp probe is on the inside of the adapter.