What Thermostat to run in an SC

My recently rebuilt SC seems to run very cool (well in the blue).
1. What thermostat did they originally come out in (Australia if that makes a difference), &
2. Would running cool impact the idle?
A few things to think about.
1. The temp gauges are not the most accurate after 50 years. Check what temperature you are actually running at with with either an infra red heat gun, or a thermometer in the top tank once the thermostat opens.
2.From factory they were equipped with 77degree c (170 degree F) thermostats. These are hard to find these days. You can either try 82 degrees, which might be a bit too hot in the summer time, or 160 degrees, which is too cool for a TC model in my experience. They tend to foul plugs even more regularly at that temperature.
3. Does your car have the crank case gases routed overboard, or into the manifold? If they go overboard then your car will run cooler, and a 180 degree thermostat will suit better,
4. Automatic tend to run hotter, 160 will suit better in my experience.

So on my Rover 2000 automatic I run a 160 degree thermostat. The other car is a TC which I run with a 180 degree thermostat. The crank case gasses go overboard.
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Thanks j_r. My car is an SC Auto (early radiator with an external oil cooler). Will try a 160 degree unit and see how I go.
Took the rover to work today and dropped in at a radiator shop on my way home. They kindly ran the thermography gun across the engine and top tank. Showed measurments between 78 and 81 degrees C. Looks like my guage is reading low.