Today we want to offer you an overview of the big-twins produced by HD, or the Milwaukee-eight (M8) family. The Harley-Davidson Milwaukee-Eight engine is the ninth generation of “big twin” engines developed by the company. But only Harley’s third all-new Big Twin in 80 years. First introduced in 2016, these engines differ from the traditional Harley Big Twin engines in that there are four valves per cylinder, totaling eight valves, hence the name.