The 45" flathead engine was made for 37 years, and was simple and efficient, if not particularly powerful..... but the real weak link in the powertrain was its three-speed transmission. These single downtube rigid frames address that issue, by using a four-speed transmission instead of the old three-speed unit. We designed a special engine sprocket for Big Twin dual row primary chains and positioned the transmission mounts so that the primary drive components and special Paughco covers from an early 74" Big Twin can be fitted to the bike. And for those who don’t want to deal with a wet clutch, we have a BDL belt drive unit instead.