The stud had forma-gasket ( the hard kind ) on it , After cleaning it up I replaced it with black RTV gasket maker. ( not sure if this stud goes into a water jacket or not and if so I am not sure if the RTV will work out ok )
The stud probably goes into the water jacket. RTV Silicone will likely seal the cooling system, but it's not the "best" product to use. RTV Silicone will shred on removal, little chunks will fall into the coolant and get circulated.
Better to use an actual thread sealer, like Loctite/Permatex 592. Fabulous stuff for sealing threads--tapered or straight.
After cleaning out the threads for the sending unit with a drill and a bore brush, I replaced the thermostat ( 195 ) and temp sending unit and the gauge now reads 1 tick higher than it did before. I can also see a more prominent swing in the gauge when the thermostat opens and closes.
Sounds great. Temp gauge may still be a little low, though.
The sending unit looks the same as the old one but the box is labeled a "switch" instead of a "sending" unit, I am not sure what difference that makes. ( if any )
As Erik the Awful said, a "switch" is intended to be on/off. Generally drives a dash warning light that would either be "out" or "full brightness".
A "sending unit" is variable-resistance, for driving a dash gauge that has a range of motion for the indicator needle.
A "sensor" can be either a switch or
(more commonly) an analog sending unit which might be a variable ground, or a variable voltage, but implies that it's signal goes to the onboard computer
instead of a "gauge" or "light" on the dash. (Not always true--but usually true.)
What does that valve in the old thermostat do, and what do the tan and black wires in the previous post go to ?
Looks like a "Jiggle Valve". Lots of import cars use those in the thermostat. Nice feature. The valve is open when the engine is shut off. Makes it nice for refilling the coolant after cooling system service; once the system is full with the air bled out, it doesn't make any difference any more.
But unlike boneheads that drill bigass holes in the thermostat flange to "let the air out", the jiggle valve CLOSES as soon as the water pump starts moving coolant. So there's no un-metered flow out of the engine and through the radiator that slows engine warm-up.
Robertshaw thermostats have a tiny opening stamped-into the flange, allows air to escape the engine during cooling system refill, but is so small that it doesn't meaningfully slow warm-up. Cheaper to stamp the flange, than to install a jiggle valve.
Typically a switch is a digital signal for a dummy light or cooling fan control. A sending unit is an analog signal for a gauge or the ECM. Unfortunately, the person printing the labels rarely has the experience of installing the part nor an understanding of the circuit.
Yup. And they likely don't speak English anyway.