Fot thise attempting this, there is a line of 22mm bungs running aling the left side of the head in all 601, 602, and 603 engines. I think the 617 has them as well. These are water gallery ports which accept fan switches. Remove one and install a fan switch. 22mm switches are available in a wide range of temperatures, from 75c to 110c. They were used in VW, Porsche, Audi, Jaguar, Alfa, and many ither Euro cars.