Primary drive

The primary drive is the first stage of power transmission, connecting the crankshaft to the clutch basket and thus the gearbox. Most modern motorcycles use a pair of gears inside the crankcase. Harley-Davidson big twins and many classic bikes use a primary chain in a separate case, and some customs use a belt.

The primary drive reduces speed: the crank turns faster than the clutch. Together with the gearbox ratios and the final drive, it sets the overall gearing.

Gear primaries need no adjustment. Primary chains stretch and need tensioner checks, and their case often has its own oil that must be changed. Whining, rattling or clunking from the clutch side can point to a worn primary drive or clutch basket.