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Recently , while on an extremely long drive realised the tempreture shot up all the way to the top and crashed out back to zero . The factors which I did see was the basic tempreture outside its self was 40 degrees Celsius. That's fine still but post that we decided anyways to repair the tempreture gauge which obviously had reached its limit . Gasket check and compression all ok. Radiator was cleaned also . Timing checked

Realised the vacuum advance had blown and was replaced with another one. A new waterpump also has been fitted and works well

Now the mystery after all that the car still in cool weather and even monsoon heats up at about 90-100 and obviously this is not good and we need switch on the extra fan . Even the tuning has been checked . We have checked with three assorted temp gauges and don't know what to do and what could be a cause.

Do guide or share tips what we should do

Thank youn

When you had he waterpump off did you remove and check/clean the water distribution tube in the block? If the heat riser valve is present, is it opening when engine warms up or is it stuck closed? Any restriction (other than the heat riser) in the muffler or exhaust pipes?

Honestly not checked that at all . Just replaced it

Need to be checked now

Thank you

If you operate outside the normal design parameters, expect issues. This is a vintage auto, not a modern rally car. You may need to stop occasionally to let things cool rather than drive on, out of range. You may need to modify/adapt your selection of lubricants, coolants, etc.
I wouldn't necessarily be concerned seeing 100 C coolant temperature, if the outside temperature is as you describe. They're pressure cooling systems, albeit low,~7 psig, so can operate slightly higher than atmospheric BP(100C)
Energy travels from hot to cold, so it has to overcome the increased ambient conditions by increasing temperature to expel heat. This is normal.
Gauge working/accuracy is a separate issue.