If you want to get really fancy:  A standard O-ring (which this is not)
is normally compressed around 20%.  So if you measure the thread pitch on
the sensor and the thickness of the O-ring, you can figure out how much to
tighten the sensor after it touches the O-ring in order to get 20% compression.
I would definitely put some coolant or something on the O-ring to allow it to
turn easily.

tabe johnson / 87 westy