Layshaft (Countershaft)

The layshaft is one of the two main shafts in a motorcycle gearbox. It runs parallel to the mainshaft (input shaft) and carries the matching gears of each gear pair. Power enters through the clutch on one shaft and leaves via the other, depending on the gearbox design.

In American usage the layshaft is often called the countershaft, and on most modern bikes it is the output shaft that carries the front sprocket, hence the term countershaft sprocket.

Layshaft bearings and the countershaft seal behind the front sprocket are typical wear points. An oil leak behind the front sprocket usually points to a worn countershaft seal, which is cheap to replace but needs the sprocket removed.