Mainshaft (gearbox input shaft)

A motorcycle gearbox usually has two parallel shafts. The mainshaft (input shaft) carries the clutch at one end and receives drive from the crankshaft through the primary drive. The countershaft (output shaft, layshaft) carries the matching gears and the front sprocket or the drive to the shaft final drive.

Gear pairs on the two shafts are always in mesh; the shift forks slide dogs or gears to lock the chosen pair to its shaft. Note that the mainshaft is often confused with a driveshaft, which is the propeller shaft taking power to the rear wheel on shaft-drive bikes.

Worn mainshaft bearings, damaged splines under the clutch basket or chipped gear dogs show up as noise, play or jumping out of gear. Fixing them normally requires splitting the crankcases.