This oversize water pump pulley is stock on late model cars, but will fit all models. Designed to reduce the horsepower drain and slow the water moving thru the radiator to allow it time to cool it's very important for the higher RPMs run today. This oversize pulley is 4.75" diameter. Use of a high capacity water pump helps to reduce cavitation in the water jacket, particularly at high RPM, as well as circulating a greater volume of coolant. Further reduction of cavitation is achieved by using a large diameter water pump pulley
CAM6408 as originally fitted to the Cooper S and reintroduced on the A-Plus engines. It is necessary to check clearance between the large pulley and the head; if a lot of material has been machined from the head face it may foul the pulley.
Small bore engines with 12G940 heads can't use CAM6408. See CAM116