To clarify, I have a threaded hole in the engine which appears to be about 3/8" diameter. I acknowledge that a 3/8" threaded nipple will have a much larger outside diameter, but since I have a 3/8" hole I was assuming that the sensor should fit into that 3/8" hole. I was also assuming that the part descriptions I have read online are referencing the hole diameter, and the style of thread (pipe thread), not actually a much larger hole than that 3/8" diameter.
I have been very active in the assuming arena lately.