|Although the RV8 does indeed have a tendency to run hot, as any passenger during a trip on a summer day will confirm, the car should not be that hot that you Ďcan hardly bear to put your hand on the bonnetí. It does sound as though you have checked that you do not have air trapped in the system and I assume that you are certain that the water pump is operating correctly. Overheating can only be the product of poor coolant flow and it is for this reason that many of us have fitted the big bore thermostat. Frankly, I have my doubts that this makes much difference but we need all the help that we can get with this issue. Do also take note of the note from the webmaster regarding overfilling of the expansion tank. This is a common problem because it does look so very empty when the engine is cold and the instinct to add more coolant overrides the fact that the book value is correct.
With regard to the matter of the fan cutting in, this will only be of interest when driving at speeds that do not allow a good flow of air through the radiator and, of course, the fan substitutes for the lack of airflow by road speed. As Ian Q has pointed out there is a very worthwhile mod that allows the water temperature gauge to read the actual temperature of the engine. With standard wiring the gauge will only read what can best be described as on- off with the on being noted by the needle pointing at the 12:00 position, as you have already observed. A fan switch thermostat, available from Fiat dealers for about £20, that switches on the fan at 97c, instead of the standard 107c, which is far too hot, will also be a very good investment. They are very easy to fit and if you are in the area of East Anglia I will be very happy to do it for you, for free of course.
However, back to the problem. I would suggest that you begin by flushing the coolant being careful not to put very cold water into a very hot engine of course. It might be an idea to get this done properly by a qualified garage if you are mechanically uncertain.
If that fails then it is most likely the radiator. There is a better radiator available, from Clive Wheatley as Ian has already mentioned, but any good radiator repairer should be able to modify the one you have, as Michael Reavil has done.
As I write this note I do know that John Cumming is in the process of fitting a special radiator to the car of Mike Sutton and, were I in your position, I would take direct contact with John Cumming for advice. John is an extremely kind man, very generous with his knowledge, and will give you the very best advice.