How large is the temperature gradient across a small block chevy with cast iron heads? The reason I ask, we are using an aluminum air gap intake, that has four access points to the circulating water, one in each corner. The sender for the gage was at the passenger side front nearest the thermostat. The temperature switch for the electric fan was at the drivers rear corner of the intake. The 190 degree fan switch came on at 215 according to the gage. We then reversed the locations, putting the gage sender at the rear of the intake on the drivers side, the fan switch at the front of the intake on the passengers side (common with the thermostat). This time the reading was 185 when the fan switch came on. I was surprised that an aluminum intake would have that much difference in temperature with the water circulating. I am confident the fan switch is repeating with consistency the temperature trip point, it's just that the water is much hotter at the thermostat. Anyone have some detailed information on the temperature differential at different locations of the water circulating the engine?