Summer is with us once more and the Mini overheating is on everybody's mind, while the temperature gauge won't keep your engine cool at least it will let you know what is happening.
To check that it is working correctly
With a capillary type gauge (mechanical)
Remove the sender unit from the head .Fill a small pot with water and bring to a rapid boil using a propane torch be careful to keep the flame away from any fuel or body parts. Insert the sending unit bulb into the boiling water. Your gauge should read 212° F or be on the start of the "H" area of the later models. If it reads otherwise, and the gauge can be pulled apart (eg Smiths) remove the gauge from the dash, and remove the chrome bezel , Twist the rim until the slots line up, remove the glass lens (it will usually needed cleaning anyway), gently pull the needle off its shaft, and replace it at 212°. Reassemble the gauge, replace the sender unit, top up coolant.
With an electric type gauge
Remove the sender unit from the head as with the capillary gauge. Using as before a small pot with boiling water, Insert the sender unit with another wire from an earth on the car . Once again your gauge should read 212° F or be on the start of the "H" area of the later models. If the gauge can be pulled apart (eg Smiths) it too can be recalibrated in the same way as the mechanical gauge.
Reassemble the gauge, replace the sender unit, top up coolant, and you are on your way, now with confidence in what your instruments are telling you.
If you don't have a propane torch or any other way of heating the water you can remove the gauge and do all this on the kitchen stove, with an electric gauge you will need a powersource and some wiring as well.