Drive side flywheels have special keyed 6° large diameter taper that provides more contact area, increasing strength Standard S&S Cycle tapered crankpin and pinion shaft are used for ease of service Flywheels for EFI applications are machined with timing notches compatible with factory crank position sensors Assembled:Flywheels, Crankpin, Connecting rods 1977-89 style pinion gears, spacers, and nuts are required when installing S&S flywheel assemblies. For installation in 1990-99 or scratch built engines, order S&S Pinion Shaft Conversion Kit 432-621 4-1/4" stroke assembly is balanced for stock H-D 80" pistons All S&S Cycle crankcases for 1984-99 big twin engines are machined to accept a crank position sensor (CPS), and can be used with these flywheels Replaces OEM 23906-90, 23906-93B, 23906-90A 4-1/4" Stroke, 8-1/2" Diameter