A one-piece coil rail assembly containing three individual coils is used on the 4.0L engine Ignition Coil Assembly - 4.0L 6 - Cylinder Engine The coil rail must be replaced as one assembly. The bottom of the coil is equipped with 6 individual rubber boots Ignition Coil Assembly - 4.0L 6 - Cylinder Engine to seal the 6 spark plugs to the coil. Inside each rubber boot is a spring. The spring is used for an electrical contact between the coil and the top of the spark plug. These rubber boots and springs are a permanent part of the coil and are not serviced separately.