When the sensor starts to fail, it reads very hot. Later it dies. It's an ether-filled plastic tube with a wire in it. I've got an end-stage sender in my 68 right now. Doesn't run normal until the temp is pegged. I know for a fact it's not boiling.

A nice upgrade is an electrical sender and gauge. If you want to avoid aftermarket junk, a 740-series temp gauge sender screws right into the engine head, but you'll have to macgyver a gauge that works and looks right.

I made this one out of a 740 dash gauge but did not get it properly mounted.



NGlasson made a much nicer one. The photo is buried in the Gallery somewhere.