Thanks for the info. I failed to describe the sensor in my original post. The sensor looks like a temp or oil sending unit, but the threaded part just looks like about a 5/16" bolt about 1/2" long, and it doesn't penetrate the block into the water jacket (as far as I can tell.) It doesn't show any signs of being in the coolant, and the cavity it goes in definitely bottoms out. I've been in several parts stores and when I asked about a temp sending unit they bring out something that looks like what you would expect, not this, and the threads are totally different. (Larger than what I have.)