The M8 engines come with oil coolers that circulate oil to the heads with majority of the oil returning to the stock oil cooler. Harleys stock oil cooler simply is not enough to keep the M8s from overheating in traffic and city riding. The side Mount UltraCool Oil Coolers for M8s work in conjunction with the stock cooler to cool all the engine oil giving your M8 the performance of a fully liquid cooled motor.